2025 OCR GCSE Gateway Science - Chemistry A - J248 Practice Paper with Answers

The mass number counts protons and neutrons. Subtract the number of protons: \(31-15=16\) neutrons. The correct option is C.

Award 1 accuracy mark for C only. Method marks: 0. DO NOT ALLOW any other letter or more than one selected option. Working is not required and does not earn a separate mark.

The formula contains one magnesium atom, two oxygen atoms and two hydrogen atoms. Its relative formula mass is \(24+2(16+1)=58\). Relative formula mass has no unit. The correct option is A.

Award 1 accuracy mark for A only. Method marks: 0. DO NOT ALLOW any other letter or multiple selections. The calculation is explanatory; no separate credit is available for working.

A submerged spot can dissolve directly into the water in the beaker. The water surface should be below the spot so that water rises through the paper and carries the ink components along it. The correct option is D.

Award 1 accuracy mark for D only. Method marks: 0. DO NOT ALLOW A, B, C or multiple selections. No written explanation is required for the mark.

In the solid, ions are held in fixed positions in a lattice. Melting allows these charged ions to move and carry an electric current. The correct option is B.

Award 1 accuracy mark for B only. Method marks: 0. DO NOT ALLOW other letters or multiple selections. An explanation without the correct selection does not earn the mark.

A liquid has a temperature above its melting point and below its boiling point. Only B satisfies this condition: \(-18<25<76\). A and D are solids at the stated temperature; C is a gas. The correct option is B.

Award 1 accuracy mark for B only. Method marks: 0. DO NOT ALLOW other letters or more than one selected option. Comparing the temperatures earns no separate method mark.

Each chlorine atom contributes one electron. The resulting shared pair forms a single covalent bond and gives each atom access to eight outer-shell electrons. No ions are formed. The correct option is D.

Award 1 accuracy mark for D only. Method marks: 0. DO NOT ALLOW A, B, C or multiple selections. No additional mark is available for describing the shared pair.

An acid reacts with a carbonate to produce a salt, water and carbon dioxide. Here the salt is potassium chloride, and carbon dioxide forms the bubbles. The correct option is A.

Award 1 accuracy mark for A only. Method marks: 0. DO NOT ALLOW B, C, D or multiple selections. A gas test or reaction equation is not required.

The temperature falls by \(22-16=6\,^{\circ}\mathrm{C}\). The reaction takes in energy from its surroundings, cooling the solution. A reaction that takes in energy in this way is endothermic. The correct option is C.

Award 1 accuracy mark for C only. Method marks: 0. DO NOT ALLOW other letters or multiple selections. The temperature-change calculation is explanatory and earns no separate mark.

A giant ionic structure contains oppositely charged ions held together by strong electrostatic attractions, giving a high melting point. Its ions cannot move through the solid. Melting allows the ions to move and carry charge, so the molten material conducts electricity. The answer is C.

Accuracy: 1 mark for C. Method: 0 marks. ALLOW an unambiguous selection of C. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

Divide the distance travelled by the substance by the distance travelled by the solvent front: \(R_f = 27/45 = 0.60\). Both distances use the same unit, so the ratio has no unit. The answer is A.

Accuracy: 1 mark for A. Method: 0 marks. ALLOW an unambiguous selection of A. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

The mass number counts protons and neutrons together. Subtract the number of protons: \(30 - 14 = 16\) neutrons. The answer is B.

Accuracy: 1 mark for B. Method: 0 marks. ALLOW an unambiguous selection of B. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

The atom has six electrons in its outer shell, placing the element in Group 6. It has three occupied electron shells, placing it in period 3. The answer is D.

Accuracy: 1 mark for D. Method: 0 marks. ALLOW an unambiguous selection of D. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

The cloudiness comes from suspended powder particles that pass through the paper. Smaller pores can retain these particles while allowing water to pass through. A wider funnel stem does not change which particles the paper retains, and adding water does not remove them. The answer is C.

Accuracy: 1 mark for C. Method: 0 marks. ALLOW an unambiguous selection of C. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

Mass is conserved during the reaction. The combined masses of the solid and gas equal the original mass of compound. Therefore, the mass of gas is \(18.6 - 14.1 = 4.5\,\mathrm{g}\). The answer is A.

Accuracy: 1 mark for A. Method: 0 marks. ALLOW an unambiguous selection of A. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

The temperature falls because the reaction absorbs energy from its surroundings, including the solution. A reaction that takes in energy is endothermic. The answer is B.

Accuracy: 1 mark for B. Method: 0 marks. ALLOW an unambiguous selection of B. DO NOT ALLOW another option or more than one option. IGNORE working; no explanation is required.

The separation starts with filtration because the graphite is insoluble. Filter paper traps the graphite, but water and dissolved potassium nitrate pass through. Rinsing the residue removes the potassium nitrate solution left on the graphite; gentle drying then produces a dry graphite sample.

Simple distillation recovers water from the filtrate. Heat the solution in a distillation flask connected to a condenser and an open receiving vessel. Water forms vapour, which cools in the condenser and becomes liquid again. Potassium nitrate does not evaporate under these conditions, so it stays in the flask. Collect the condensed water separately.

Remove only part of the water by distillation. Stop heating when the remaining solution is concentrated enough for crystallisation, without heating it to dryness. Allow it to cool. Potassium nitrate is less soluble in cold water, so crystals form. Filter the crystals from the remaining solution and dry them between filter papers. This gives three separate samples without discarding the water as vapour.

Use a best-fit level of response to award a maximum of 6 marks. Assess the scientific method and its accuracy together; do not assign separate method marks or accuracy marks to individual statements. First select the level from the scientific content and reasoning, then select the mark within that level using clarity, relevance and logical organisation.

Level 3 (5–6 marks): A workable, logically ordered method obtains all three requested samples, including dry solids and collected liquid water. Suitable apparatus is described or shown. Explanations correctly link filtration to insolubility, distillation to evaporation and condensation, and crystallisation to reduced solubility on cooling or sufficient solvent removal. Award 6 for a clear, coherent account with sound practical detail; award 5 where there is a minor omission or some lack of clarity.

Level 2 (3–4 marks): A partly developed sequence would successfully separate at least two components, with a correct explanation for at least one separation. Apparatus or recovery details may be incomplete. Award 4 for a mostly connected and scientifically accurate account; award 3 where the account is less clear or has significant omissions.

Level 1 (1–2 marks): Identifies at least one suitable separation technique or gives relevant practical detail, but does not develop a workable sequence to obtain the samples. Award 2 for some linked, accurate detail; award 1 for an isolated relevant statement.

0 marks: No scientifically creditworthy content.

Indicative content — credit equivalent workable methods, without requiring this exact wording or order:
• Filter paper in a funnel separates graphite as the residue from potassium nitrate solution as the filtrate.
• Graphite is retained because it is insoluble; dissolved potassium nitrate passes through with the water.
• Rinse the graphite with a little clean water to remove remaining salt solution, then dry the residue gently.
• Heat the filtrate in a distillation flask connected to a cooled condenser and a separate receiving vessel. The apparatus must allow pressure to escape.
• Water evaporates, then cools and condenses; potassium nitrate remains in the flask because it does not evaporate under these conditions.
• Collect the condensed water separately and leave some concentrated solution in the flask.
• Cool the concentrated solution so potassium nitrate crystallises as its solubility decreases.
• Filter off the crystals and dry them between filter papers or by suitable gentle drying.

Guidance: ALLOW AW and suitable labelled diagrams. ALLOW a different scientifically valid sequence that recovers all three samples. ALLOW controlled evaporation to concentrate the salt solution further after some water has been collected by distillation. ALLOW washing crystals with a small amount of cold clean water before drying. IGNORE omission of a crystal-washing step. DO NOT ALLOW filtration alone as a way to remove dissolved potassium nitrate. DO NOT ALLOW evaporation in an open dish alone as a method for collecting liquid water. DO NOT ALLOW the claim that potassium nitrate evaporates and is collected in the condenser. Boiling the salt solution to dryness is not an appropriate crystallisation step. These errors limit the credit for the affected process; apply the overall level descriptors rather than subtracting marks mechanically. No calculation or error-carried-forward marks apply.

(a) Atomic number gives the proton number, so the first missing value is \(20\). Neutron number is mass number minus proton number: \(44-20=24\). A neutral atom has equal numbers of protons and electrons, giving \(20\) electrons.
(b) The number of protons defines the element. Both isotopes contain \(20\) protons even though their neutron numbers differ.
(c) The final number in \(2,8,8,2\) is the outer-shell electron count: \(2\).
(d) Losing two negatively charged electrons leaves the atom with two more protons than electrons, forming \(\mathrm{Ca}^{2+}\).
(e) The charges balance in the ratio of one calcium ion to two chloride ions: \((+2)+2(-1)=0\). The formula is therefore \(\mathrm{CaCl_2}\).

Award a maximum of 9 marks. Each tick represents one mark.

| Part | Credit | Guidance |
| --- | --- | --- |
| (a) | First blank: \(20\) ✓; second blank: \(24\) ✓; third blank: \(20\) ✓. [3 accuracy marks] | Mark each cell independently. No working is required. |
| (b) | Same proton number / both have \(20\) protons ✓. [1] | ALLOW same atomic number. DO NOT ALLOW same mass number or same electron number alone. |
| (c) | \(2\) ✓. [1] | Accept words or numeral. |
| (d) | Electrons are lost ✓; exactly two electrons are lost ✓. [2] | Award both for “loses two electrons”. Award the second mark only when loss of electrons is identified. DO NOT ALLOW loss of protons. “Gains two electrons” scores zero. |
| (e) | \(\mathrm{CaCl_2}\) ✓; one \(+2\) ion balances two \(-1\) ions / total positive and negative charges are equal for the stated \(1:2\) ratio ✓. [2] | Mark formula and explanation independently. ALLOW a correct charge-sum calculation for the explanation. DO NOT ALLOW \(\mathrm{Ca_2Cl}\) or \(\mathrm{CaCl}\) as the formula. |

No ECF applies. Accept scientifically equivalent wording (AW).

(a) Each formula unit contains one potassium ion and one chloride ion. Add the two relative atomic masses: \(M_r=39.0+35.5=74.5\). Relative formula mass has no unit.
(b) Scientific purity means that the material contains only a single element or a single compound.
(c) Water selectively dissolves potassium chloride, leaving graphite undissolved. Stirring helps the salt dissolve. Filter paper traps the graphite particles while the dissolved salt passes through with the water. The filtrate is therefore the solution to keep. Gentle heating evaporates some water and concentrates this solution. Cooling the concentrated solution produces potassium chloride crystals. Filtering separates the crystals from the remaining liquid; pressing gently between filter papers removes liquid from their surfaces. Eye protection reduces the risk from splashes, and tongs and a heatproof mat help prevent burns or damage from hot apparatus. The solution should not be heated to dryness.

Maximum 9 marks: (a) 2, (b) 1, (c) 6.

| Part | Credit | Guidance |
| --- | --- | --- |
| (a) | \(39.0+35.5\) ✓ [1 method mark]; \(74.5\) ✓ [1 accuracy mark]. | Check the answer first: award 2 for \(74.5\), even without working. If the answer is incorrect, award the method mark for correct addition set up. IGNORE an appended unit. No ECF. |
| (b) | Contains only one element or one compound ✓. [1] | ALLOW one chemical substance only / no other substances mixed with it. DO NOT ALLOW “clean”, “safe” or “natural” alone. |

(c)* Use best-fit levels, not a point-counting scheme. Scientific coverage and explanations establish the level. Clarity, relevance and logical organisation determine the mark within that level.

Level 3 (5–6 marks): A workable, logically sequenced method covers selective dissolving, filtration with the correct fraction retained, concentration and crystallisation, and collection and drying of crystals. Explains why dissolving and filtration separate the components and how crystals are obtained. Includes a suitable heating precaution. Award 6 for a clear, well-linked account; award 5 for a minor omission or less clear explanation.

Level 2 (3–4 marks): A substantially correct sequence describes dissolving the salt and removing graphite, followed by a plausible route towards crystals. At least one separation step is explained. Some apparatus, crystal recovery detail or safety detail may be missing. Award 4 for the more complete and coherent response; award 3 for a partially developed response.

Level 1 (1–2 marks): Some relevant apparatus or steps are identified, but the sequence is incomplete or contains errors that prevent a reliable preparation. Explanations are limited or absent. Award 2 for more than one linked, useful detail; award 1 for an isolated creditworthy detail.

0 marks: No creditworthy scientific content.

Indicative content, not an exhaustive checklist:
• Add water in a beaker and stir; potassium chloride dissolves but graphite does not.
• Use a filter funnel and filter paper, collecting the filtrate in a suitable vessel.
• Graphite is retained as residue; dissolved potassium chloride passes through.
• Heat the filtrate gently in an evaporating basin to evaporate some water and make a concentrated solution.
• Stop heating and allow the concentrated solution to cool so crystals form.
• Filter off the crystals and dry them between filter papers or in a warm place.
• Wear eye protection; use a heatproof mat, avoid vigorous boiling, or handle hot apparatus with suitable tongs.

ALLOW careful slow evaporation at room temperature as a route to crystals; a suitable precaution for heating must still be given because the question requests one. ALLOW other scientifically sound ways to collect and dry crystals. DO NOT ALLOW filtering the dry mixture as an effective separation. DO NOT ALLOW retention of the initial graphite residue as the source of potassium chloride crystals. Heating to complete dryness does not demonstrate controlled crystallisation. Apply these notes within the best-fit judgement; do not subtract separate penalty marks.

(a) At \(130\,{}^{\circ}\mathrm{C}\), sulfur is above its melting point and below its boiling point, so it is liquid. Sodium chloride is below its melting point, so it is solid.
(b) Some electrons in copper are delocalised. These electrons can move through the metal and transfer electrical charge when a potential difference is applied.
(c) Sodium chloride contains charged ions in both states. In the solid they are held in a lattice and cannot move through the material. Melting allows the ions to move and carry charge, so the molten compound conducts electricity.
(d) In diamond, each carbon atom bonds to four other carbon atoms, forming a giant covalent network. The covalent bonds throughout this network are strong. A large amount of energy must be supplied to break bonds and break down the structure, so a very high temperature is required.

Award a maximum of 9 marks. Each tick represents one mark.

| Part | Credit | Guidance |
| --- | --- | --- |
| (a) | Sulfur is liquid ✓; sodium chloride is solid ✓. [2] | Mark independently. No explanation is needed. |
| (b) | Copper has delocalised / free electrons ✓; these electrons move through the metal and carry charge ✓. [2] | ALLOW electron flow for the second point if movement of charge is clear. “Electrons move and carry charge through the copper” can earn both marks because mobility implies free electrons. DO NOT ALLOW moving positive ions as the charge carriers in solid copper. |
| (c) | In molten sodium chloride the ions move and carry charge ✓; in solid sodium chloride the ions occupy fixed positions and cannot move through the solid ✓. [2] | ALLOW “free to move when molten but not when solid” if explicitly referring to ions. DO NOT ALLOW electrons moving through molten sodium chloride or ions being absent in the solid. |
| (d) | Diamond has a giant covalent structure / a continuous covalently bonded network ✓; its covalent bonds are strong ✓; breaking these bonds requires much energy ✓. [3] | ALLOW a network of carbon atoms each bonded to four others for the structure point. DO NOT ALLOW strong forces between separate diamond molecules. “Many bonds” alone does not earn the strong-bond point. IGNORE descriptions of electrical conductivity. |

These are knowledge, application and explanation marks; no calculation method marks or ECF apply. Accept equivalent scientific wording (AW).

(a) Water dissolves the sodium chloride but leaves the sand undissolved. Stirring helps the salt dissolve. Filter paper retains the sand while the salt solution passes through. Rinsing the residue removes salt solution trapped among the sand grains. Drying removes water from the sand. Collecting the washings with the filtrate retains the dissolved salt. Gentle heating in an evaporating basin removes water and concentrates the solution. Leave the concentrated solution to cool and crystallise, then filter and blot the crystals dry. A small test drop forming crystals on cooling can indicate when sufficient water has evaporated. Eye protection limits injury from splashes; tongs allow hot apparatus to be handled safely. Careful evaporation to dryness is an alternative way to recover salt.

(b)(i) Subtract the empty dish mass from the combined mass:
\(m_{\text{salt}}=49.80-42.60=7.20\,\mathrm{g}\).

(b)(ii) Compare the collected salt mass with the starting mixture mass:
\(\text{percentage}=\dfrac{7.20}{18.0}\times100=40\%\).
This is the collected mass as a percentage of the starting mixture. It does not assume that all the salt originally present was recovered.

(c) The sample is impure. A pure substance has a sharp melting point. The sample melts over a range, and this range is lower than the stated melting point of pure sodium chloride.

17(a)* [6 marks: level of response]
Use best fit. Scientific completeness and reasoning establish the level. Clarity, relevance and logical organisation determine the mark within the level.

Level 3 (5–6 marks): Gives a workable sequence producing both separate, dry solids with appropriate apparatus. Explains the roles of dissolution, filtration and water removal. Includes a relevant heating precaution. Award 6 for a complete, clearly justified account; award 5 for minor omissions or less clear reasoning.
Level 2 (3–4 marks): Describes most of a workable sequence, including dissolution, filtration and a valid way of recovering salt. Gives some scientific explanation, but drying, apparatus, safety or other details may be incomplete. Award 4 for a more coherent and developed account.
Level 1 (1–2 marks): Gives relevant operations or facts, but the account would not reliably produce both separate, dry samples. Award 2 for more developed relevant content.
0 marks: No creditworthy response.

Indicative content, not a checklist: add water in a beaker and stir; salt dissolves but sand does not; filter paper and funnel retain sand; collect the salt solution; rinse sand to remove remaining salt solution; combine washings with filtrate; dry sand; heat filtrate gently in an evaporating basin; evaporation removes water; cool a concentrated solution to form crystals; filter and dry crystals; eye protection, gentle heating to limit spitting, or tongs for hot apparatus. ALLOW careful evaporation to dryness as an alternative salt-recovery method. ALLOW a suitable water bath or electric heater. DO NOT ALLOW filtering the initial dry mixture alone as a complete separation. Do not allocate one mark per indicative point.

Part | Credit | Guidance
17(b)(i) [1] | A1: \(7.20\,\mathrm{g}\) ✓ | ALLOW 7.2. Unit is supplied. No separate method mark.
17(b)(ii) [2] | M1: \(\text{mass from (b)(i)}\div18.0\times100\) ✓ A1: \(40\%\) ✓ | Check the final answer first: a correct answer earns both marks. ALLOW 40.0%. ECF: award both marks for correct substitution and evaluation using the student's numerical answer from (b)(i), provided the denominator is 18.0 and multiplication by 100 is used. DO NOT ALLOW division by dish mass.
17(c) [2] | Impure ✓ Melts over a range / melts below \(801\,{}^{\circ}\mathrm{C}\) ✓ | Require a relevant comparison with pure sodium chloride or its expected sharp melting point for the explanation mark. ALLOW either the range or the lower temperature; both are not required. DO NOT ALLOW 'pure because it melts'.

Total: \(6+1+2+2=11\) marks.

(a) P melts below \(20\,{}^{\circ}\mathrm{C}\) and boils above this temperature, so it is liquid. Q and R both have melting points above \(20\,{}^{\circ}\mathrm{C}\), so they are solid.

(b) R has the conductivity pattern of an ionic compound. Its charged ions occupy fixed positions in the solid lattice, so they cannot flow through the material. Melting allows the ions to move and carry an electric current.

(c) Q contains delocalised electrons. These can move through the solid metal and carry charge; movement of metal ions is not needed.

(d) Boiling separates molecules of P. The attractions between its molecules are weak, so overcoming them requires little energy. The covalent bonds within each molecule remain intact.

(e) Layers in a metallic structure can slide relative to one another. Attraction between positive metal ions and delocalised electrons continues as the layers move, keeping the material bonded in its new shape.

Part | Credit | Guidance
21(a) [2] | P: liquid; Q: solid; R: solid ✓✓ | Award 2 marks for all three correct; 1 mark for exactly two correct; otherwise 0. ALLOW unambiguous state symbols. Do not award one mark per entry.
21(b) [3] | R ✓ Solid ions are fixed in position / cannot move through the lattice ✓ Liquid ions can move and carry charge ✓ | Award points independently. ALLOW 'molten' for liquid. DO NOT ALLOW movement of electrons as the conduction mechanism in liquid R. 'Particles move' is insufficient unless identified as ions.
21(c) [2] | Contains delocalised / free electrons ✓ These electrons move through the structure and carry charge ✓ | ALLOW AW. 'Electrons move through the metal and carry charge' implies both points and may earn both marks. DO NOT ALLOW movement of metal ions as the conduction mechanism in solid Q.
21(d) [2] | Weak forces of attraction between molecules ✓ Little energy is needed to overcome these forces and separate the molecules ✓ | The energy statement must link to overcoming intermolecular forces; repeating the question alone earns no mark. ALLOW weak intermolecular bonds if distinguished from covalent bonds. DO NOT ALLOW weak covalent bonds within molecules or breaking covalent bonds as the cause of boiling.
21(e) [2] | Layers of metal ions / atoms slide past each other ✓ Metallic bonding is maintained by attraction between positive ions and delocalised electrons ✓ | ALLOW layers moving relative to each other. ALLOW 'metallic bonds remain as layers slide' for the second point. DO NOT ALLOW layers held together only by weak intermolecular forces. A labelled diagram may supply either credit point if clear.

These are discrete scientific accuracy marks; no calculation method marks or ECF apply. IGNORE irrelevant statements unless they contradict a credited point.
Total: \(2+3+2+2+2=11\) marks.

(a) Dissolving the sodium chloride allows the two solids to be separated because their solubilities differ. Add enough water and stir so that all the salt dissolves. Pour the mixture through filter paper held in a funnel over a beaker. Insoluble sand is trapped on the paper, while dissolved salt passes through with the water. Rinse the sand with a small amount of water to remove salt solution trapped between the grains. Collect these washings with the filtrate. Dry the sand in a warm place or by gentle heating.
Transfer the filtrate to an evaporating basin. Heat gently to evaporate some water until the solution is sufficiently concentrated to form crystals on cooling. Stop heating and allow it to cool. Filter the crystals from the remaining solution and pat them dry between filter papers. Wear eye protection because hot solution may splash. Let heated apparatus cool before handling it, or use tongs.

(b) Since the mixture has only two components, subtract the sand mass from the total mass:
\(m_{\text{sodium chloride}}=24.0-7.2=16.8\,\mathrm{g}\).
Divide the sodium chloride mass by the original mixture mass and multiply by 100:
\(\text{percentage}=\frac{16.8}{24.0}\times100=70\%\).

(c) Dissolving is a physical change. No new substance is formed, and sodium chloride can be recovered by removing water.

(d) Electrical conduction requires charged particles that can move through a material. Sodium chloride contains positive and negative ions. In solid sodium chloride these ions occupy fixed positions in a lattice, so they cannot carry charge through the solid. Water separates the ions when sodium chloride dissolves. The ions can then move through the solution and carry charge.

(a)* [6] Level-of-response marking. Use best fit; indicative points are not a six-point checklist. Scientific completeness and linked reasoning determine the level. Within a level, use clarity, relevance and logical organisation to choose the mark.
Level 3 (5–6 marks): A workable, logically ordered method obtains both dry sand and dry salt crystals. Describes dissolution, filtration, concentration/crystallisation and drying, with appropriate apparatus. Explains how solubility and filtration or evaporation enable separation. Includes a suitable heating precaution. Award 6 for a clear, well-connected account; 5 for a less complete or less clearly communicated account meeting this level overall.
Level 2 (3–4 marks): Describes a broadly workable sequence involving dissolution and filtration, followed by removal of water to obtain salt. Gives some appropriate apparatus or explanatory reasoning. Details of drying, crystallisation or safe heating may be incomplete. Award 4 for a coherent account; 3 for a less developed account.
Level 1 (1–2 marks): Gives relevant practical steps or scientific ideas, but the account is incomplete or would not reliably obtain both dry products. Award 2 for linked relevant ideas; 1 for an isolated creditworthy idea.
0 marks: No creditworthy response.
Indicative content: add sufficient water and stir to dissolve salt; sand stays insoluble; filter using a funnel and filter paper over a suitable collecting vessel; sand is the residue and salt solution is the filtrate; rinse sand and combine washings with filtrate; dry sand; gently heat solution in an evaporating basin to remove some water; stop heating when sufficiently concentrated; cool to form crystals; filter and dry crystals; wear eye protection against splashes; use tongs or allow hot apparatus to cool.
Guidance: ALLOW a valid alternative method that gives both dry products, including careful gentle evaporation to dryness. ALLOW sensible alternative drying methods. Washing the sand is desirable but is not compulsory for Level 3. DO NOT ALLOW the claim that filter paper removes dissolved sodium chloride. IGNORE unspecified statements such as 'be careful' as safety evidence. Serious errors that prevent separation should be reflected in the best-fit level.

(b) [3] Check the final answer first: award 3 marks for \(70\%\), with or without working.
Credit | Guidance
M1: \(24.0-7.2\) to find sodium chloride mass, giving \(16.8\,\mathrm{g}\). ✓ | Credit the correct subtraction method even if its evaluation contains an arithmetic slip.
M1: sodium chloride mass divided by \(24.0\), multiplied by \(100\). ✓ | ALLOW ECF using the candidate's sodium chloride mass from an attempted subtraction. Do not award this method mark for using the sand mass directly.
A1: \(70\%\). ✓ | ALLOW \(70.0\%\). This accuracy mark requires the correct answer; no ECF. The percentage symbol is supplied on the answer line, so do not require it to be repeated.

(c) [2]
Credit | Guidance
B1: Physical change. ✓ | DO NOT ALLOW both physical and chemical unless the incorrect choice is clearly cancelled.
B1: No new substance forms / sodium chloride can be recovered unchanged by removing the water. ✓ | ALLOW AW. Credit the scientific reason independently. IGNORE 'it dissolves' alone; DO NOT ALLOW 'the salt disappears' as the reason.

(d) [3]
Credit | Guidance
B1: Sodium chloride contains charged ions / positive and negative ions that can carry charge. ✓ | A reference to ions is sufficient to identify charged particles; 'particles' alone is insufficient.
B1: In the solid the ions are fixed in position and cannot move through the solid. ✓ | ALLOW ions are held in a lattice. DO NOT ALLOW the claim that solid sodium chloride has no ions.
B1: In solution the ions can move and carry charge through the liquid. ✓ | ALLOW free-moving ions conduct electricity. DO NOT ALLOW conduction attributed to free electrons in the solution.

Total: 14 marks. M = method; A = numerical accuracy; B = independent scientific credit. ECF applies only where explicitly stated.

First consider how the track forms. In graphite, each carbon atom is covalently bonded to three other carbon atoms within a layer. The forces between layers are weak, so layers can slide over one another and rub off onto the rough card. This leaves graphite on the card.

Next consider electrical conduction. One outer electron from each carbon atom in graphite is delocalised. These electrons can move through the continuous graphite track and carry electrical charge.

Compare this with diamond. Each carbon atom is covalently bonded to four other carbon atoms, producing a rigid three-dimensional network of strong bonds. This makes diamond hard, so it would not rub off readily to leave a track. All four outer electrons of each carbon atom are involved in bonding. There are no delocalised electrons to move through diamond, so even a continuous diamond track would not conduct electricity. The two properties needed here are therefore both explained by the differences in structure and bonding.

Use a best-fit level-of-response judgement. Award one holistic mark out of 6; do not award separate method or accuracy marks or count indicative points as individual marks. Scientific accuracy, completeness and linked reasoning determine the level. Clarity, relevance and logical organisation determine the mark within that level.

Level 3 (5–6 marks): Gives a coherent comparison linking structure and bonding to both track formation and electrical conduction for graphite and diamond. Explains graphite rubbing off through weak forces between layers and its conduction through mobile delocalised electrons. Explains diamond resisting rubbing off through its rigid network of strong covalent bonds and its lack of conduction through the absence of mobile electrons. Award 6 for a clear, accurate and logically connected explanation; award 5 where the explanation meets this level but has a minor omission or lacks clarity in places.

Level 2 (3–4 marks): Gives some linked explanations using correct structural or bonding ideas. Explains both relevant properties of one material, or explains one relevant property for both materials. Comparison or scientific detail is incomplete. Award 4 for clearer and more developed links; award 3 for a less developed explanation that meets this level.

Level 1 (1–2 marks): Gives relevant properties or structural facts about graphite or diamond, with limited or absent links explaining suitability. Award 2 for several accurate relevant statements or a simple explanatory link; award 1 for an isolated creditworthy statement.

0 marks: No creditworthy scientific content.

Indicative content (other valid explanations may be credited):
• Graphite contains carbon atoms arranged in layers; each carbon atom forms three covalent bonds.
• Strong covalent bonds hold atoms together within a layer.
• Weak forces between layers allow layers to slide and rub off onto the card.
• Each carbon atom in graphite contributes one delocalised electron.
• Mobile delocalised electrons carry charge along the graphite track.
• Diamond has four covalent bonds per carbon atom in a giant three-dimensional network.
• Strong covalent bonds throughout this rigid network make diamond hard and resistant to rubbing off.
• All outer electrons in diamond are involved in bonding; there are no delocalised electrons to carry charge.

Guidance: ALLOW free electrons for delocalised electrons when their movement is clear. ALLOW weak attractions between layers or layers separating for the graphite explanation. ALLOW alternative wording (AW) that conveys the same scientific links. DO NOT ALLOW weak covalent bonds within graphite layers as the reason graphite rubs off. DO NOT ALLOW moving carbon atoms or ions as the charge carriers. DO NOT ALLOW diamond having no electrons as an explanation of non-conduction. IGNORE cost, appearance or claims that diamond scratches the card unless accompanied by relevant structure–property reasoning. Do not require a diagram, an equation or a particular order of explanation.

A metal displaces a less reactive metal from its salt solution. The first result places magnesium above zinc; the second places zinc above copper. The order from most reactive to least reactive is magnesium, zinc, copper, so C is correct.

Accuracy/selection: 1 mark for C (magnesium, zinc, copper). Method: 0 marks. ALLOW an unambiguous selection of C. DO NOT ALLOW any other option or more than one option. No working is required.

Iron(II) ions form a green precipitate of iron(II) hydroxide with sodium hydroxide solution. Iron(III) gives a brown precipitate, copper(II) gives a blue precipitate and zinc gives a white precipitate. Therefore A is correct.

Accuracy/selection: 1 mark for A (iron(II)). Method: 0 marks. ALLOW an unambiguous selection of A. DO NOT ALLOW any other option or more than one option. No working is required.

Substitute the collected mass as the actual yield: \(\text{percentage yield}=\dfrac{6.3}{9.0}\times100=70\%\). Therefore D is correct. The missing fraction of the product is \(30\%\), which is not the yield.

Accuracy/selection: 1 mark for D (\(70\%\)). Method: 0 marks. ALLOW an unambiguous selection of D. DO NOT ALLOW any other option or more than one option. No working is required.

Mean rate is the volume produced divided by the time taken: \(\text{mean rate}=\dfrac{72}{45}=1.6\,\mathrm{cm^3/s}\). Therefore B is correct. Dividing time by volume gives the reciprocal of the required rate.

Accuracy/selection: 1 mark for B (\(1.6\,\mathrm{cm^3/s}\)). Method: 0 marks. ALLOW an unambiguous selection of B. DO NOT ALLOW any other option or more than one option. No working is required.

A volumetric pipette is calibrated to deliver its stated fixed volume accurately. A \(25.0\,\mathrm{cm^3}\) pipette is therefore suitable for transferring this sample. The measuring cylinder is less accurate, and the markings on beakers and conical flasks give only approximate volumes. A is correct.

Accuracy/selection: 1 mark for A (the volumetric pipette). Method: 0 marks. ALLOW an unambiguous selection of A. DO NOT ALLOW any other option or more than one option. No working is required.

Propene is an alkene and contains a carbon–carbon double bond. During addition polymerisation, this double bond opens so that many propene molecules can join to form a chain. The other substances listed have no carbon–carbon double bond. Therefore C is correct.

Accuracy/selection: 1 mark for C (propene). Method: 0 marks. ALLOW an unambiguous selection of C. DO NOT ALLOW any other option or more than one option. No working is required.

Q and S have densities below \(2.0\,\mathrm{g/cm^3}\). Q fails the breaking-force requirement because \(25\,\mathrm{N}\) is below \(40\,\mathrm{N}\). S has a breaking force of \(65\,\mathrm{N}\), so it meets both requirements. P and R are too dense. Therefore D is correct.

Accuracy/selection: 1 mark for D (material S). Method: 0 marks. ALLOW an unambiguous selection of D. DO NOT ALLOW any other option or more than one option. No working is required.

Zinc is more reactive than iron, so it is oxidised in preference to the iron while the metals remain in contact. This is sacrificial protection. A scratch breaks the physical barrier provided by the coating, but the zinc can still provide sacrificial protection. Therefore B is correct.

Accuracy/selection: 1 mark for B (zinc reacts in preference to iron). Method: 0 marks. ALLOW an unambiguous selection of B. DO NOT ALLOW any other option or more than one option. No working is required.

A metal displaces a less reactive metal from its salt solution. X is therefore more reactive than Y, and Z is more reactive than X. The order is Z, X, Y, so B is correct.

1 accuracy mark for B. Method marks: 0. Accept only B as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

Iron(II) ions form a green precipitate of iron(II) hydroxide with sodium hydroxide. Calcium ions give a white precipitate, copper(II) ions give a blue precipitate and iron(III) ions give a brown precipitate. D is correct.

1 accuracy mark for D. Method marks: 0. Accept only D as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

Mean rate is the volume produced divided by the time taken: \(\text{mean rate}=72\div48=1.5\,\mathrm{cm^3/s}\). A is correct. Dividing time by volume gives the wrong quantity; subtracting or adding the two measurements does not calculate a rate.

1 accuracy mark for A. Method marks: 0. Accept only A as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

Percentage yield compares the actual mass with the theoretical maximum: \(\text{percentage yield}=\frac{9.0}{12.0}\times100=75\%\). C is correct. The missing mass represents \(25\%\), not the yield.

1 accuracy mark for C. Method marks: 0. Accept only C as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

A straight-chain alkene with one carbon–carbon double bond has the general formula \(\mathrm{C_nH_{2n}}\). With four carbon atoms, it has eight hydrogen atoms, giving \(\mathrm{C_4H_8}\). C is correct. The other formulae are those of alkanes.

1 accuracy mark for C. Method marks: 0. Accept only C as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

Zinc is more reactive than iron. While it remains in contact with the iron, zinc oxidises preferentially and provides sacrificial protection, including at the scratch. B is correct. The coating does not need to form a complete physical barrier for this protection to work.

1 accuracy mark for B. Method marks: 0. Accept only B as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

Viewing a burette scale at an angle causes parallax error. Placing the eyes level with the bottom of the meniscus allows it to be aligned correctly with the scale. D is correct. Adding liquid more slowly can help judge the end-point, but it does not correct the viewing error.

1 accuracy mark for D. Method marks: 0. Accept only D as the single selected option. Do not allow another option or more than one selected option. No explanation is required.

(a) An acid reacting with a carbonate produces a salt, water and carbon dioxide.

(b) The decrease in mass equals the mass of gas that escapes:
\(154.82 - 154.34 = 0.48\,\mathrm{g}\).
The mean rate is:
\(\dfrac{0.48}{40} = 0.012\,\mathrm{g/s}\).

(c) A higher acid concentration means more acid particles in each unit volume. These particles collide successfully with the chip surfaces more often, increasing the initial reaction rate.

(a) [1]
Carbon dioxide ✓ (1 knowledge mark).
Guidance: ALLOW \(\mathrm{CO_2}\). DO NOT ALLOW carbon monoxide.

(b) [2]
Award 2 marks for \(0.012\,\mathrm{g/s}\), even without working.
Otherwise award:
M1: Calculates the mass decrease as \(154.82 - 154.34 = 0.48\,\mathrm{g}\) ✓.
A1: Divides the mass decrease by \(40\,\mathrm{s}\) to obtain \(0.012\,\mathrm{g/s}\) ✓.
Guidance: The answer-line unit is supplied; no separate unit mark. ALLOW equivalent numerical forms. ECF: if the mass subtraction is incorrect, award the second mark for correctly dividing that positive mass decrease by 40; maximum 1 mark in this case. DO NOT ALLOW dividing the total mass of the flask and contents by the time.

(c) [2]
More acid particles per unit volume ✓ (1 explanation mark).
Successful collisions between acid particles and the chip surfaces occur more frequently ✓ (1 explanation mark).
Guidance: ALLOW more frequent collisions with the calcium carbonate without the word 'successful'. ALLOW hydrogen ions for acid particles and equivalent wording (AW). DO NOT ALLOW 'more particles' alone without a volume comparison. DO NOT ALLOW particles moving faster or having more energy as an explanation of the concentration effect. IGNORE references to a greater final gas volume.

Total: 5 marks.

(a)(i) First find the mass saved: \(1.68-0.56=1.12\ \mathrm{kg}\). Use the original steel shelf mass as the denominator: \(\text{percentage decrease}=\frac{1.12}{1.68}\times100=66.666\ldots\%\). To one decimal place, this is \(66.7\%\).

(a)(ii) Multiply the alloy mass by its price per kilogram: \(0.56\times4.50=2.52\). The material costs £2.52.

(b) Pure aluminium has regularly arranged layers of atoms. Atoms of another element in the alloy have a different size, disrupting this arrangement. The layers cannot slide past each other as easily, so the alloy is harder.

(c) Zinc is above iron in the reactivity series. The zinc reacts in preference to the iron and is gradually used up. This sacrificial protection works while the remaining zinc is in contact with the steel, even though the scratch has broken the physical barrier.

Total: 8 marks.

| Part | Credit points | Guidance |
|---|---|---|
| (a)(i) | ✓ M1: mass decrease \(1.68-0.56=1.12\ \mathrm{kg}\). ✓ M1: divides the mass decrease by \(1.68\) and multiplies by \(100\). ✓ A1: \(66.7\%\), to one decimal place. [3] | Check the final answer first: award all 3 marks for \(66.7\%\), even without working. ALLOW an equivalent combined calculation. If the mass decrease is calculated incorrectly, ALLOW ECF for the second method mark when that value is divided by \(1.68\) and multiplied by \(100\); the final accuracy mark requires \(66.7\%\). An otherwise correct unrounded answer earns 2 marks. DO NOT ALLOW division by the new shelf mass for the second method mark. |
| (a)(ii) | ✓ A1: £2.52. [1] | ALLOW \(252\) pence if clearly identified. This is an independent calculation; no ECF from (a)(i). |
| (b) | ✓ Atoms of different sizes disrupt/distort the regular layers or arrangement. ✓ This makes it more difficult for layers of atoms to slide past each other. [2] | ALLOW AW. Award the second point independently for a correct statement about restricted sliding in the alloy. DO NOT ALLOW atoms themselves becoming harder. IGNORE vague statements that the alloy is stronger. |
| (c) | ✓ Zinc is more reactive than iron. ✓ Zinc reacts/oxidises/corrodes in preference to iron, so iron is protected. [2] | ALLOW zinc loses electrons more readily for the first point. ALLOW zinc acts as a sacrificial metal and is used up instead of iron for the second point. DO NOT ALLOW a barrier-only explanation: the iron at the scratch is exposed. IGNORE the claim that zinc itself rusts if the intended preferential corrosion is otherwise clear. |

(a) A higher temperature gives particles more kinetic energy. Faster movement increases collision frequency. It also increases the proportion of collisions with energy at least equal to the activation energy, so more collisions lead to reaction.

(b) Measure a fixed volume of dilute hydrochloric acid from one stock solution. Weigh a fixed mass of calcium carbonate chips chosen to have similar sizes in every run. These controls keep acid concentration, acid volume, chip mass and exposed surface area comparable.

Set a water bath to \(18\,^{\circ}\mathrm{C}\). Place the acid in the flask and allow it to reach this temperature; bring the chips to the same temperature separately, keeping them dry. Check the temperature with a thermometer. Connect the gas syringe to the delivery tube and ensure that its plunger moves freely. Add the chips to the acid, start the stopclock and fit the bung promptly. Keep the flask in the bath during the reaction. Use the same mixing and sealing procedure each time to minimise differences in gas lost at the start.

Read the gas volume every \(10\ \mathrm{s}\), for example over the first \(60\ \mathrm{s}\). Compare gas production over an identical short initial interval, chosen so that none of the reactions has finished during that interval. Calculate a mean rate over that interval using \(\text{mean rate}=\frac{\text{gas volume collected}}{\text{time taken}}\), in \(\mathrm{cm^3\,s^{-1}}\). The larger value indicates the faster reaction.

Use fresh reactants and repeat the procedure at \(28\,^{\circ}\mathrm{C}\) and \(38\,^{\circ}\mathrm{C}\). Obtain at least three results at each temperature. Investigate an anomalous result by checking the apparatus and repeating the run; calculate a mean from consistent repeat results. Compare the means to establish how temperature affects rate. Wear eye protection to reduce the risk from acid splashes and secure the gas syringe.

Total: 8 marks.

(a) Point-based marking [2]

| Credit points | Guidance |
|---|---|
| ✓ Particles move faster, leading to more frequent collisions. [1] | ALLOW more collisions per second. DO NOT ALLOW faster movement alone without the link to collision frequency for this point. |
| ✓ A greater proportion of particles/collisions have sufficient energy for reaction. [1] | ALLOW more collisions exceed the activation energy. DO NOT ALLOW the claim that heating lowers activation energy. Award independently of the first point. |

(b)* Level-of-response marking [6]
Use best fit. Establish the level from the scientific completeness and reasoning. Within that level, award the higher mark for a clear, relevant and logically organised response. A diagram may supply evidence. Indicative points are not a compulsory sequence or a point-by-point checklist.

| Level | Marks | Descriptor |
|---|---|---|
| 3 | 5–6 | A workable and well-controlled investigation establishes and maintains the different temperatures and measures gas production with time. It explains a valid comparison of rates and how repeats improve confidence in the results. The sequence and links between measurements, controls and conclusions are clear. |
| 2 | 3–4 | A generally workable method changes temperature and measures gas production with time. Some relevant variables are controlled. Use of results or treatment of reliability is outlined, but there are omissions in temperature control, fair comparison or detail. |
| 1 | 1–2 | Some relevant apparatus, measurements or controls are identified. The method is incomplete or gives little explanation of how to make a valid comparison of rates. |
| 0 | 0 | No creditworthy response. |

Indicative content:
• Select the same mass and similar sizes of chips for each run, keeping exposed surface area comparable.
• Use the same acid volume and concentration, with fresh reactants for each trial.
• Use water baths at the stated temperatures; allow reactants to reach the selected temperature and check with a thermometer.
• Keep the reaction flask in the bath to maintain its temperature.
• Collect carbon dioxide in a gas syringe through an airtight bung and delivery tube; check for leaks and a freely moving plunger.
• Start timing on mixing and seal promptly, using a consistent procedure.
• Record volumes at regular times, or measure the time needed to collect a fixed gas volume.
• Compare volumes produced during the same initial interval, or calculate volume divided by time. Alternatively, a shorter time to collect the same volume indicates a faster rate.
• Choose an interval or target volume that allows a fair comparison before any reaction has finished.
• Repeat at every temperature, identify possible anomalies, repeat questionable runs and calculate means from consistent results.
• Compare the mean results, or plot mean rate against temperature, to identify the trend.
• Wear eye protection because acid could splash; secure the syringe or give another relevant precaution with a reason.

ALLOW a valid method based on measuring mass loss with time if it includes suitable apparatus, temperature control and a valid comparison. ALLOW reasonable alternative timing intervals and repeat numbers. DO NOT ALLOW final gas volume alone as a measure of reaction rate. DO NOT ALLOW changing acid concentration, chip mass or chip size as part of the temperature comparison. IGNORE unrelated precautions. No separate method/accuracy marks or numerical ECF apply to this level-of-response task.

(a)(i) The volume increase is \(54-0=54\ \mathrm{cm^3}\) over \(45-0=45\ \mathrm{s}\). Therefore, \(\text{mean rate}=54/45=1.2\ \mathrm{cm^3/s}\).

(a)(ii) Pass some of the collected gas through limewater. Carbon dioxide makes limewater cloudy because an insoluble solid forms.

(b) Weigh equal masses of large chips and small chips. For each experiment, measure the same volume of dilute hydrochloric acid of the same concentration. Use enough acid for it to remain in excess. Start both experiments at the same temperature. These controls allow chip size to be the variable being tested.

Connect a conical flask to a gas syringe using a bung and delivery tube. Check that the apparatus is airtight and that the syringe moves freely. Add one sample to the acid, start the stopwatch and fit the bung immediately. Use the same mixing and bung-fitting procedure in every run to limit differences in gas lost at the start. Record the gas volume at fixed intervals, for example every \(10\ \mathrm{s}\), until the volume stops increasing. Repeat with the other chip size and fresh acid.

Perform at least three runs for each size. Investigate unusual readings and repeat a run if needed, then calculate mean gas volumes at each time. Compare the means at an early time when both reactions are still occurring. Producing more gas in this time indicates a faster reaction. The final gas volume alone does not compare rates: equal masses of pure calcium carbonate with excess acid should produce the same total volume.

Wear eye protection to protect against splashes of dilute acid.

(a)(i) [2]: Check the final answer first. Award both marks for \(1.2\) in the given answer space, even without working. Otherwise: M1 for substituting \(54/45\) or an equivalent correct change-in-volume/change-in-time calculation ✓ A1 for \(1.2\ \mathrm{cm^3/s}\) ✓. The unit is supplied on the answer line; no separate unit mark. ALLOW \(1.20\). DO NOT ALLOW the inverted calculation. No ECF is needed.

(a)(ii) [1]: Bubble/pass the gas through limewater AND state that it turns cloudy/milky or forms a white precipitate ✓. Both the test and observation are required for this one mark. ALLOW shake the gas with limewater. DO NOT ALLOW a glowing-splint or lighted-splint test. IGNORE a correct explanation of precipitate formation; it earns no additional mark.

(b)* [6]: Use best-fit level-of-response marking. Establish the level from the scientific quality, completeness and reasoning. Within the level, use clarity, relevance and logical organisation to choose the mark. The indicative content is not a checklist of six separate marks.

Level 3 (5–6 marks): Gives a workable method to collect comparable rate data for both chip sizes. Explains the principal controls and how repetition improves the evidence. Explains a valid way to identify the faster reaction from the measurements. Includes an appropriate safety precaution. Award 6 for a clear, logically organised and sufficiently detailed account; award 5 where there are minor omissions or less developed links.

Level 2 (3–4 marks): Describes a method that could produce useful rate measurements and identifies some relevant controls. Gives some consideration to reliable results or to interpreting the measurements. The method or explanations have gaps. Award 4 for a generally clear account with several linked points; award 3 for a less developed account.

Level 1 (1–2 marks): States a few relevant practical steps, controls or observations, but does not develop a workable comparison of rates. Award 2 for more than one relevant idea presented with some clarity; award 1 for limited relevant content.

0 marks: No creditworthy content.

Indicative content:
• Vary chip size while using calcium carbonate of the same purity and equal mass.
• Keep acid concentration, acid volume and starting temperature the same; use excess acid. Keep mixing consistent.
• Use a flask, airtight bung and delivery tube connected to a gas syringe; check for leaks and free syringe movement.
• Start timing when reactants meet and minimise initial gas loss consistently. ALLOW a suitable arrangement that mixes the reactants after the apparatus has been sealed.
• Measure gas volume at regular times, or measure the time to collect a fixed volume below the final volume for both samples.
• Repeat each condition and calculate suitable means; investigate anomalous readings and repeat if necessary. Repeats reduce the effect of random variation.
• A greater gas volume in the same early time, a shorter time for the same gas volume, or a steeper initial volume–time graph indicates a faster rate.
• Wear eye protection to reduce the risk from acid splashes.

Guidance: ALLOW a valid alternative rate method, such as recording mass loss over time using a balance with sufficient resolution and a loose cotton-wool plug to reduce spray while allowing gas to escape. Apply the same expectations for controls, repeats and interpretation. ALLOW collecting gas over water with suitable apparatus. DO NOT ALLOW comparing final gas volumes alone as evidence for different rates. DO NOT ALLOW equal numbers of differently sized chips as a substitute for equal masses. Merely naming a gas syringe or stopwatch without saying what is measured is insufficient for a developed method. IGNORE predictions about which size reacts faster unless linked to a valid interpretation of measurements. No ECF applies to this part.

Total: \(2+1+6=9\) marks.

(a) Carbon dioxide turns limewater milky because an insoluble calcium carbonate precipitate forms. Pass the collected gas through limewater to observe this change.

(b) Divide the gas volume by the elapsed time:
\(\text{mean rate}=\frac{36}{45}=0.80\text{ cm}^3/\text{s}\).
This is the average rate during the first \(45\text{ s}\), rather than the rate at a single instant.

(c) Weigh an equal mass of chips from each size group. Measure an equal volume of the same dilute hydrochloric acid for every experiment and keep the starting temperature constant. Choose quantities that leave acid in excess and produce less gas than the syringe capacity.

Place the acid in a conical flask connected by an airtight bung and delivery tube to a gas syringe. Keep the chips in a small dry container inside the flask, separate from the acid. Tip the sealed flask to mix the reactants and start the timer immediately. This avoids losing gas while fitting a bung after the reaction starts. Record the syringe reading at regular time intervals. Treat all three chip sizes in the same way and wear eye protection when handling the acid.

Use a preliminary experiment to select an early comparison time, before any size group has finished reacting. Repeat the experiment at least three times with fresh reactants for each size. Investigate any anomalous result by repeating that trial, then calculate mean gas volumes at the comparison time. The prediction is supported if smaller chips consistently give a larger mean volume in this same time. Comparing final gas volumes alone would not measure the difference in rate.

For an equal mass of marble, small chips expose a larger total surface area. More acid particles can collide with the marble surface each second. At the same temperature, the greater frequency of successful collisions produces a faster reaction.

(a) [2]
Credit point | Guidance
Pass/bubble the gas through limewater. ✓ [1] | ALLOW adding limewater to a collected gas sample and shaking. DO NOT ALLOW simply adding limewater to the acid–marble reaction mixture.
Limewater becomes cloudy/milky or forms a white precipitate. ✓ [1] | The observation mark depends on naming limewater. IGNORE statements about the gas having no colour. DO NOT ALLOW a colour change without cloudiness or precipitate.

(b) [2: 1 method mark, 1 accuracy mark]
Credit point | Guidance
Substitutes \(36\div45\). ✓ M1 | ALLOW an equivalent calculation using correctly converted units.
Obtains \(0.80\text{ cm}^3/\text{s}\). ✓ A1 | Check the final answer first: a correct answer earns both marks without working. ALLOW \(0.8\). Units are supplied on the answer line; no separate unit mark. If the final answer is wrong, award M1 for a correct substitution. No ECF from an inverted calculation.

(c)* [6: level of response]
Use best fit. Scientific completeness and reasoning determine the level. Within a level, select the higher mark when the response is clear, relevant and logically organised; select the lower mark when communication or precision is less secure. Do not award one mark for each item in the indicative content.

Level 3 [5–6]: Presents a workable, fair investigation yielding numerical rate comparisons, with a suitable procedure for repeats and processing results. Explains the prediction using increased exposed surface area and increased frequency of successful collisions. The method and explanation form a coherent response.

Level 2 [3–4]: Gives a broadly workable method for comparing rates with some appropriate controls. Includes either a developed step to improve reliability or a linked surface-area/collision explanation. Some detail needed for a fair, reliable comparison or a full explanation is missing.

Level 1 [1–2]: Gives relevant but limited experimental suggestions or scientific statements. The method is insufficiently developed to support a fair numerical comparison, and/or the explanation has little linkage.

[0]: No creditworthy response.

Indicative content — not an exhaustive checklist:
• Use chips from the same marble source, sorted into distinct size groups.
• Keep marble mass, acid volume, acid concentration and starting temperature the same; use acid in excess.
• Use a balance for chip mass and suitable apparatus to measure acid volume.
• Collect gas using a conical flask, airtight connections and a gas syringe of adequate capacity; begin timing when the reactants mix and minimise initial gas loss.
• Record gas volumes at fixed intervals. Compare volumes after the same early time, before any experiment has finished, or calculate mean rates over the same suitable interval.
• Alternatively, measure the time to collect a fixed gas volume that every trial can reach; a shorter time indicates a faster reaction.
• Repeat each chip size, identify results that need checking and calculate means. Use fresh reactants each time.
• State how the processed data would support or contradict the prediction.
• Wear eye protection and handle dilute acid carefully.
• Equal masses of smaller chips have a larger total exposed surface area.
• Acid particles collide with the solid surface more frequently, increasing successful collisions per second.

Guidance: ALLOW a valid mass-loss method using a balance, a loosely fitted cotton-wool plug and measurements over time, with the same controls and suitable comparison. ALLOW AW throughout. DO NOT ALLOW comparing only the final gas volume as evidence of rate. DO NOT ALLOW smaller chips making particles move faster or lowering activation energy. IGNORE an unlinked statement that particles are closer together. A labelled diagram may supply method details but is not required. Safety details strengthen the plan but their omission alone does not impose a level cap.

Total: \(2+2+6=10\) marks.

(a) Hydrochloric acid forms chloride salts. The calcium in calcium carbonate therefore forms calcium chloride.

(b) Pass a sample of the gas through limewater. Carbon dioxide makes limewater cloudy because an insoluble solid forms.

(c) Divide the volume collected by the collection time:
\(\text{mean rate} = \dfrac{54}{45} = 1.2\,\text{cm}^3/\text{s}\).

(d) Measure gas production using a conical flask, bung, delivery tube and gas syringe. Check that the connections are airtight so that the gas produced is collected. Measure the same volume of each acid solution and use the same mass of calcium carbonate granules from the same size range. Use a small enough mass of carbonate for even the most dilute acid to be in excess. Keep the starting temperature the same: temperature and the exposed surface area of the carbonate can also affect rate.

Add the granules, fit the bung immediately and start the timer. Use the same procedure each time to minimise differences in gas lost at the start. Record the gas volume at regular times. Choose a short initial time interval during which all the reactions are still producing gas. Calculate each mean rate by dividing the gas volume collected during this interval by its duration. A larger volume in the same time means a higher mean rate.

Carry out at least three trials at each concentration. Investigate an unexpected result by repeating the experiment, then calculate a mean using consistent results. Repetition reduces the effect of random variation. Wear eye protection because hydrochloric acid could splash into the eyes.

(a) [1]
Credit: calcium chloride ✓ (1 accuracy mark).
Guidance: ALLOW \(\mathrm{CaCl_2}\). DO NOT ALLOW calcium chlorate or calcium chlorine.

(b) [2]
Credit: bubble/pass the gas through limewater ✓ (1 method mark); limewater turns milky/cloudy, or a white precipitate forms ✓ (1 observation mark).
Guidance: ALLOW shaking a collected sample of gas with limewater. Award the observation mark only when linked to limewater. IGNORE additional correct descriptions of the precipitate. DO NOT ALLOW a glowing or lighted splint as the carbon dioxide test.

(c) [2]
Check the answer first: \(1.2\,\text{cm}^3/\text{s}\) gains both marks.
Otherwise, credit: \(54 \div 45\) ✓ (1 method mark); correctly evaluated as \(1.2\) ✓ (1 accuracy mark).
Guidance: Units are supplied on the answer line; no separate unit mark. ALLOW equivalent numerical forms such as \(1.20\). If the division is correct but the arithmetic is wrong, award the method mark only. DO NOT ALLOW \(45 \div 54\) for the method mark. No ECF from other parts applies.

(d)* [6: level of response]
Use best fit. Scientific completeness and reasoning determine the level; clarity, relevance and logical organisation determine the mark within that level. Do not award one mark per indicative point.

Level 3 (5–6 marks): A workable, logically organised investigation compares several acid concentrations using valid rate measurements. Relevant controls are identified and their purpose is explained. The response explains how repetition or another suitable step improves reliability and includes an appropriate safety precaution. The procedure is sufficiently detailed to carry out.

Level 2 (3–4 marks): A generally workable method varies concentration and measures gas production over time. Some relevant controls or reliability steps are described, with some scientific reasoning. Details of timing, comparison, reliability or safety may be incomplete.

Level 1 (1–2 marks): Some relevant apparatus, measurements or procedural steps are given. The response provides limited detail about how a fair or reliable rate comparison would be made.

0 marks: No creditworthy response.

Indicative content:
• Use several known acid concentrations, recording the concentration for each experiment.
• Collect gas in a gas syringe connected to a flask by an airtight bung and delivery tube; check connections for leaks.
• Measure equal acid volumes and use equal masses of carbonate granules from the same size range.
• Keep starting temperature constant. Explain that temperature and exposed surface area also affect reaction rate.
• Use enough acid for it to be in excess at all concentrations.
• Start timing on mixing and fit the bung promptly, using a consistent procedure to limit differences in initial gas loss.
• Measure gas volume at regular time intervals.
• Compare volumes during the same short initial interval, before any reaction has finished, or calculate volume divided by time for this interval. More gas per unit time indicates a faster reaction.
• Repeat trials at each concentration and calculate a mean. Investigate anomalies with further trials. Explain that repetition reduces the effect of random variation.
• Wear eye protection to protect against acid splashes.

Guidance: ALLOW other valid methods, such as measuring loss of mass at timed intervals using a balance and a loose cotton-wool plug to reduce splashing while allowing gas to escape. ALLOW comparison of times to collect a fixed gas volume that every reaction can produce; a shorter time indicates a higher rate. For such alternatives, judge controls, repeat measurements and reasoning on their scientific merits. DO NOT credit total final gas volume alone as a measurement of rate. Do not require every indicative point for Level 3. IGNORE an unsupported prediction about which concentration reacts fastest: the task assesses the investigation plan.

Total: \(1 + 2 + 2 + 6 = 11\) marks.

(a) An acid reacting with a carbonate forms a salt, water and carbon dioxide. The gas is carbon dioxide.

(b) Add the three measured times and divide by the number of trials:
\(\text{mean time}=\dfrac{46+50+48}{3}=\dfrac{144}{3}=48\text{ s}\).
The same gas volume is collected in every trial, so use the requested volume divided by the mean time:
\(\text{mean rate}=\dfrac{36}{48}=0.75\text{ cm}^3\text{/s}\).

(c) Crushing the same mass exposes more calcium carbonate surface to the acid. More acid particles can collide with the solid each second. More successful collisions occur per second, so the gas is produced faster.

(d) One suitable plan is as follows. Measure \(40\text{ cm}^3\) of the first acid solution using a measuring cylinder. Weigh \(0.20\text{ g}\) of calcium carbonate chips. Use a conical flask fitted with an airtight bung and a delivery tube leading to a freely moving gas syringe. To avoid losing gas at the start, keep the acid in a small upright container inside the flask, separate from the chips. Fit the bung, then tip the flask to mix the reactants while starting a stopwatch. Record the gas volume every \(5\text{ s}\) for the first \(20\text{ s}\). A preliminary trial should confirm that the reaction has not finished within this interval at any concentration; use a shorter common interval if needed.

Repeat this procedure for each known acid concentration. Keep the acid volume, mass of calcium carbonate, chip size and initial temperature constant. Use the same apparatus and mixing procedure. Carry out at least three trials per concentration using fresh reactants. Check any unusual result with an additional trial and calculate a mean gas volume for each time. Compare the mean gas volumes after the same initial interval, or divide these volumes by that interval to compare mean rates. A greater volume in the same time indicates a faster reaction. Wear eye protection because dilute acid can splash into the eyes.

Total: 12 marks.

(a) [1]
Credit | Guidance
✓ Carbon dioxide. (1 accuracy mark) | ALLOW \(\mathrm{CO_2}\). DO NOT ALLOW carbon monoxide. No gas test is required.

(b) [3]
Check the final answers first. Award all three marks for \(48\text{ s}\) and \(0.75\text{ cm}^3\text{/s}\), even without working.
Credit | Guidance
✓ Mean time \(=(46+50+48)/3=48\text{ s}\). (1 accuracy mark) | ALLOW 48 on the labelled answer line.
✓ Substitution \(36/48\) into volume divided by time. (1 method mark) | ALLOW \(36/t\), where \(t\) is the candidate's positive calculated mean time from these three trials. DO NOT ALLOW time divided by volume.
✓ Mean rate \(=0.75\text{ cm}^3\text{/s}\). (1 accuracy mark) | ALLOW equivalent precision. ECF: award this mark for correctly evaluating \(36/t\) using the candidate's positive incorrect mean time, provided volume divided by time is used. Units are supplied on the answer line; no separate unit mark. A correct rate without a mean time or working earns the two rate marks only.

(c) [2]
Credit | Guidance
✓ Powder has a greater exposed surface area than an equal mass of chips. (1 explanation mark) | ALLOW a greater surface-area-to-volume ratio. DO NOT ALLOW larger particles or a greater mass as the explanation.
✓ More frequent collisions between acid particles and the calcium carbonate surface give more successful collisions per second. (1 explanation mark) | ALLOW more acid particles colliding with the solid each second, AW. DO NOT ALLOW particles moving faster or having more energy because the solid was crushed. IGNORE references to total gas volume unless they contradict the credited explanation.

(d)* [6, level of response]
Use best fit across the whole response; do not award one mark per listed feature. Scientific completeness and reasoning determine the level. Clarity, relevance and logical organisation determine the mark within that level.

Level 3 (5–6 marks): A coherent, workable plan compares the supplied concentrations using appropriate timed measurements. Relevant controls make the comparison valid, and the response explains how results will be processed or compared to establish rates. It addresses reliability and safe working. Award 6 for clear, well-connected detail; award 5 where a minor omission does not undermine the investigation.

Level 2 (3–4 marks): A broadly workable method changes acid concentration and measures reaction progress with time. Some relevant controls or explanation of comparison are included. Detail about repeat measurements, processing, safe working or operation of the apparatus may be incomplete. Award 4 for the more developed, logically organised response.

Level 1 (1–2 marks): Some relevant apparatus, measurements or controls are identified, but the method is incomplete or does not yet support a valid comparison. Award 2 when relevant ideas are linked; award 1 for an isolated creditworthy idea.

Level 0 (0 marks): No creditworthy response.

Indicative content; no fixed sequence or wording is required:
• Use all three known acid concentrations and record which solution is used.
• Measure equal acid volumes with a measuring cylinder; the stated \(40\text{ cm}^3\) is suitable.
• Use a balance for equal chip masses; the stated \(0.20\text{ g}\) is suitable.
• Select chips with similar sizes; keep initial temperature and the mixing procedure constant.
• Collect gas using a conical flask, bung, delivery tube and gas syringe, or an inverted water-filled measuring cylinder over water.
• Start timing as the reactants mix; minimise initial gas loss. A sealed arrangement allowing separate reactants to be mixed is suitable.
• Record gas volume at regular times, using an initial interval during which all reactions are still taking place. Compare volumes over the same interval or calculate volume divided by elapsed time.
• Alternatively, measure time to reach the same small gas volume and compare times or calculate that volume divided by each time. A shorter time indicates a faster reaction.
• Repeat each concentration with fresh reactants, check anomalous results and calculate suitable means.
• Wear eye protection to protect against acid splashes.

ALLOW other scientifically valid methods, including a sufficiently sensitive balance to measure mass loss over time, with a loose cotton-wool plug to reduce spray while allowing gas to escape. ALLOW an appropriate water bath to control temperature. DO NOT ALLOW comparing only final gas volumes as a measure of rate. DO NOT ALLOW a sealed rigid vessel with no outlet for the gas as a workable collection arrangement. A prediction about which concentration reacts fastest is not required. There are no separate method or accuracy marks within this level-of-response task.