2026 Cambridge IGCSE Science - Combined (0653) Practice Paper with Answers

(a) (i) Chemical digestion of protein starts in the stomach.
(ii) Proteases (such as pepsin) break down proteins into amino acids.
(iii) Hydrochloric acid creates an acidic environment (optimum pH for gastric protease) and destroys pathogens/bacteria ingested with food.

(b) (i) To test for protein, add Biuret reagent (or dilute sodium hydroxide followed by dilute copper(II) sulfate). A positive result shows a colour change from blue to purple/lilac.
(ii) To test for reducing sugars, add Benedict's solution to the food sample and heat in a water bath (e.g. at 80 °C) for several minutes. A positive result is indicated by a colour change from blue to green, yellow, orange, or brick-red precipitate depending on concentration.

(c) Proteins always contain nitrogen in addition to carbon, hydrogen, and oxygen (some also contain sulfur).

(a)(i) stomach; [1]

(a)(ii) protease; [1]
ALLOW pepsin / peptidase

(a)(iii) kills bacteria / kills pathogens / destroys microorganisms;
OR provides optimum / acidic pH (for protease / pepsin activity);
OR denatures harmful enzymes in food; [1]
DO NOT ACCEPT just 'digests food'

(b)(i) add Biuret (reagent / solution) / add aqueous sodium hydroxide AND aqueous copper(II) sulfate; [1]
(colour turns) purple / lilac / violet; [1]
ALLOW mauve
DO NOT ACCEPT blue to pink alone unless purple is mentioned

(b)(ii) add Benedict's (solution / reagent); [1]
heat / place in a (hot) water bath / warm above 60–80 °C; [1]
(colour turns / gives) green / yellow / orange / red / brick-red (precipitate); [1]
DO NOT ACCEPT heating without mention of reagent

(c) nitrogen; [1]
ALLOW N
ALLOW sulfur / S

(a) Photosynthesis combines carbon dioxide and water to produce glucose and oxygen:
\(6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2\)

(b) (i) Xylem is the specialised vascular tissue responsible for the transport of water and dissolved mineral ions.
(ii) Water enters root hair cells from the soil by osmosis. It passes across the root cortex into the xylem vessels, travels up the stem, and enters the mesophyll cells of the leaf. Water evaporates from the surfaces of the mesophyll cells into the intercellular air spaces and diffuses out into the atmosphere through open stomata.

(c) Higher wind speed removes accumulated water vapour from the boundary layer around the stomata. This maintains a steep water vapour concentration gradient between the air spaces inside the leaf and the external air, increasing the rate of diffusion of water vapour out of the leaf.

(d) Increased temperature increases the kinetic energy of water molecules, leading to faster evaporation and higher transpiration rates. (Alternatively: decreased humidity or increased light intensity).

(a)
- \(\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2\) correct formulas for reactants and products [1];
- correct balancing: 6, 6 -> 1, 6 [1];

(b) (i)
- xylem / xylem vessels [1];
(ii)
- water enters root hair (cells) / roots (by osmosis) [1];
- travels up the xylem (vessels in stem / leaf) [1];
- evaporates from mesophyll (cells) and diffuses out through stomata [1];

(c)
- moves / blows water vapour away from the leaf surface / stomata [1];
- maintains / increases the concentration gradient (of water vapour) / faster diffusion of water vapour [1];

(d)
- (increased) temperature / (decreased) humidity / (increased) light intensity [1];
ALLOW warmer air / drier air / lower moisture in air
DO NOT ACCEPT just 'sunlight' without reference to intensity or brightness.

(a) (i) A pathogen is defined as a disease-causing organism.

(ii) Suitable barriers include:
- Mechanical barriers: skin, hairs in the nose, cilia (in the respiratory tract).
- Chemical barriers: mucus, stomach acid (hydrochloric acid in gastric juice), lysozyme/tears.

(b) Bacteria are cellular organisms with structures such as cell walls, bacterial ribosomes, and distinct metabolic pathways that antibiotics specifically inhibit or disrupt. Viruses do not have cellular structures or cell walls; they reproduce inside the host's own cells using the host's metabolic machinery, making them unaffected by antibiotics.

(c) (i)
1. The vaccine introduces harmless (dead, weakened, or fragmented) pathogens containing specific antigens into the body.
2. White blood cells (lymphocytes) recognise the antigens and produce specific antibodies against them.
3. Memory cells are produced and remain in the body, ensuring that if the live pathogen infects the person later, antibodies are produced much more rapidly and in greater quantity, preventing illness (active immunity).

(ii) When a large majority of the population is vaccinated, the transmission of the pathogen is greatly reduced (herd immunity), meaning unvaccinated individuals are far less likely to encounter an infected host.

(a)(i)
- a disease-causing organism / organism that causes disease [1]
[ALLOW micro-organism / named type of pathogen]

(a)(ii) Any two for [1] mark each:
- skin [1]
- nose hairs [1]
- mucus [1]
- cilia / ciliated cells [1]
- stomach acid / hydrochloric acid in stomach / gastric juice [1]
- tears / lysozyme [1]
[MAX 2]

(b)
- antibiotics target bacterial cellular structures / bacterial cell walls / bacterial metabolism / kill living cells [1]
- viruses do not have cells / do not have cell walls / reproduce inside host cells [1]

(c)(i)
- vaccine introduces antigens / dead / weakened / inactive pathogen [1]
- stimulates (lymphocytes / white blood cells) to produce (specific) antibodies [1]
- memory cells are produced (which give long-term defence / rapid response on re-infection) [1]

(c)(ii)
- herd immunity / fewer infected people to transmit the disease / lower probability of non-vaccinated person encountering the pathogen [1]

(a)(i) Number of neutrons = nucleon number - proton number = 7 - 3 = 4.
(a)(ii) With 3 electrons, the electronic configuration is 2, 1.
(b) When lithium reacts with fluorine:
- The lithium atom loses its 1 outer-shell electron to form a positive lithium ion (\(\text{Li}^+\)).
- The fluorine atom gains this 1 electron to complete its outer shell and form a negative fluoride ion (\(\text{F}^-\)).
- The oppositely charged ions (\(\text{Li}^+\) and \(\text{F}^-\)) attract each other by strong electrostatic forces.
(c)(i) An electrolyte is a molten or aqueous substance / ionic compound that conducts electricity and is decomposed in the process.
(c)(ii) Positive lithium ions (\(\text{Li}^+\)) move to the negative cathode where they gain electrons to form lithium metal.
(c)(iii) Negative fluoride ions (\(\text{F}^-\)) lose electrons at the anode to produce fluorine gas; observed as bubbles / effervescence / pale green-yellow gas.
(c)(iv) In solid lithium fluoride, ions are held in fixed positions in a giant ionic lattice and cannot move. In molten lithium fluoride, the lattice breaks down and the ions are free to move and carry charge.

(a)(i) 4 [1];
(a)(ii) 2, 1 / 2.1 [1];
(b) Lithium (atom) loses one electron / outer electron [1];
Fluorine (atom) gains one electron [1];
(Strong) electrostatic attraction between oppositely charged ions / between \(\text{Li}^+\) and \(\text{F}^-\) [1];
(c)(i) A molten or aqueous substance / ionic compound that conducts electricity and is broken down / decomposed by it [1];
(c)(ii) Lithium / \(\text{Li}\) [1];
(c)(iii) Bubbles / effervescence / gas given off / pale yellow-green gas [1]; ALLOW yellow gas / green gas; IGNORE smell;
(c)(iv) Ions are free to move in molten (liquid) state but are fixed / cannot move in solid [1]; DO NOT ACCEPT electrons moving for ionic conduction.

(a) The reactants are magnesium (\(\text{Mg}\)) and hydrochloric acid (\(\text{HCl}\)). The products are magnesium chloride (\(\text{MgCl}_2\)) and hydrogen gas (\(\text{H}_2\)).
Balanced equation: \(\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\)

(b)
- In an exothermic reaction, the products have lower chemical energy than the reactants, so the products line must be drawn below the reactants line.
- The activation energy, \(E_\text{a}\), is the minimum energy required to start the reaction; it is represented by an arrow extending upwards from the reactant energy level to the peak of the curve.
- The overall energy change (\(\Delta H\)) is represented by an arrow extending from the reactant energy level down to the product energy level.

(c)(i) Using powder increases the rate of reaction (makes the reaction faster).

(c)(ii) Magnesium powder has a larger surface area (or greater surface area to volume ratio) than magnesium ribbon. This means more magnesium particles are exposed, leading to a greater frequency of collisions (more collisions per unit time/second) between magnesium atoms and hydrogen ions/acid particles.

(a)
- Correct formulas of reactants and products (\(\text{Mg}\), \(\text{HCl}\), \(\text{MgCl}_2\), \(\text{H}_2\)) [1]
- Correct balancing: \(\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\) [1]

(b)
- Products line drawn horizontal and lower than the reactants line [1]
- Activation energy, \(E_\text{a}\), indicated by an arrow pointing upwards from the reactant level to the highest point of the pathway curve [1]
- Overall energy change, \(\Delta H\), indicated by a vertical arrow pointing downwards from the reactant level to the product level [1]
(ALLOW double-headed arrow if clearly labelled; DO NOT ACCEPT arrows starting from axis/baseline)

(c)(i)
- Rate increases / reaction is faster [1]
(ALLOW time taken decreases; IGNORE reaction finishes quicker without reference to rate/speed)

(c)(ii)
- (Powder has a) larger / greater surface area (or more particles exposed) [1]
- Higher frequency of collisions / more collisions per second / more collisions per unit time [1]
(DO NOT ACCEPT 'more collisions' without a time reference such as 'per second' / 'frequency')

(a) Natural gas is predominantly composed of methane (\(\text{CH}_4\)).

(b) A homologous series is a family of similar compounds with:
1. The same general formula (e.g., \(\text{C}_n\text{H}_{2n+2}\) for alkanes).
2. Similar chemical properties.
3. A trend in physical properties (such as increasing boiling point with chain length).
4. Successive members differing by a \(-\text{CH}_2-\) unit.

(c)(i) Cracking requires thermal energy (high temperature, typically 450–800 °C) and/or a catalyst (such as zeolite, silica, or alumina).
(c)(ii) Total carbon atoms = 10; product has 2 in ethene, leaving \(10 - 2 = 8\) carbons. Total hydrogen atoms = 22; product has 4 in ethene, leaving \(22 - 4 = 18\) hydrogens. Hence, the missing hydrocarbon is octane, \(\text{C}_8\text{H}_{18}\).

(d) Aqueous bromine (bromine water) tests for unsaturation (C=C double bonds).
- When shaken with ethane (saturated), the solution remains orange/brown.
- When shaken with ethene (unsaturated), the solution is rapidly decolourised (turns from orange to colourless).

(e) Incomplete combustion (combustion in a limited supply of oxygen) produces carbon monoxide (\(\text{CO}\)). Carbon monoxide binds tightly to haemoglobin in red blood cells, which prevents oxygen from binding and thereby reduces the oxygen-carrying capacity of the blood.

(a) methane [1]

(b) any two from:
- same general formula [1]
- same functional group [1]
- similar chemical properties / same chemical reactions [1]
- trend / gradual change in physical properties [1]
- consecutive members differ by \(\text{CH}_2\) [1]

(c)(i) high temperature / heat / 450–800 °C OR catalyst / zeolite / aluminium oxide / silica-alumina [1]
(c)(ii) \(\text{C}_8\text{H}_{18}\) [1]

(d)
- (aqueous) bromine / bromine water [1]
- (ethane:) remains orange / brown / yellow / no change AND (ethene:) turns colourless / decolourises [1]
[DO NOT ACCEPT 'turns clear' for colourless]

(e)
- carbon monoxide / \(\text{CO}\) [1]
- binds with haemoglobin / reduces blood's ability to carry/transport oxygen [1]
[IGNORE 'causes breathing difficulties' unless linked to haemoglobin / oxygen transport]

(a)(i)
\[W = mg = 450\text{ kg} \times 1.6\text{ N/kg} = 720\text{ N}\]

(a)(ii)
\[\text{Resultant force } F = \text{Thrust} - \text{Weight} = 1170\text{ N} - 720\text{ N} = 450\text{ N}\]
Direction: upwards / in the direction of the thrust.

(a)(iii)
Using \(F = ma\):
\[a = \frac{F}{m} = \frac{450\text{ N}}{450\text{ kg}} = 1.0\text{ m/s}^2\]

(b)(i)
Using \(v = \frac{2\pi r}{T}\):
\[v = \frac{2 \times \pi \times 1.8 \times 10^6\text{ m}}{7.2 \times 10^3\text{ s}} = \frac{1.131 \times 10^7}{7200} \approx 1570\text{ m/s} \quad (\text{or } 1.6 \times 10^3\text{ m/s})\]

(b)(ii)
Gravitational force / gravity / gravitational attraction.

(c)(i)
Gravitational attraction / gravity pulls the particles of gas and dust together.

(c)(ii)
Nuclear fusion (hydrogen fusing to form helium).

(a)(i) [1 mark]
720 (N) ;

(a)(ii) [2 marks]
450 (N) ; [1]
Upwards / vertically up ; [1] (ALLOW ecf from (a)(i))

(a)(iii) [2 marks]
(a =) F / m OR 450 / 450 ; [1]
1.0 m/s² (or 1 m/s²) ; [1] (ALLOW ecf from (a)(ii))

(b)(i) [2 marks]
(v =) 2 × π × r / T OR (2 × π × 1.8 × 10⁶) / (7.2 × 10³) ; [1]
1570 (m/s) OR 1600 (m/s) OR 1.57 × 10³ (m/s) ; [1]

(b)(ii) [1 mark]
Gravitational force / gravity / gravitational attraction ;

(c)(i) [1 mark]
Gravity / gravitational force pulls (clouds of) gas and dust together ;

(c)(ii) [1 mark]
(Nuclear) fusion / hydrogen fusing into helium ;

(a)(i) Gas particles are in continuous, random motion. When they collide with the inner walls of the container, they exert a force on the area of the wall, creating pressure (\(p = \frac{F}{A}\)).

(a)(ii) As the temperature increases, the particles gain kinetic energy and move faster. This causes them to collide with the walls more frequently and with greater force/momentum, resulting in an increased total force per unit area, and hence higher pressure.

(b)(i) Sound is a longitudinal wave. The particles of the medium vibrate/oscillate parallel to (along) the direction of energy transfer (direction of wave propagation).

(b)(ii) Using the wave equation:
\(v = f \lambda\)
\(\lambda = \frac{v}{f} = \frac{340\text{ m/s}}{850\text{ Hz}} = 0.40\text{ m}\)

(a)(i)
- (gas) particles in continuous / random motion [1];
- particles collide with / hit the walls (of the cylinder), exerting a force (per unit area) [1];
ALLOW: molecules / atoms for particles.
DO NOT ACCEPT: particles colliding with each other as the cause of pressure on the wall.

(a)(ii)
- particles have more kinetic energy / move faster / have higher average speed [1];
- (particles) collide with the walls more frequently / more often OR collide with greater force / harder [1];
DO NOT ACCEPT: particles expand.

(b)(i)
- longitudinal [1];
- particles vibrate / oscillate parallel to (the direction of) wave travel / energy transfer [1];
ALLOW: back and forth in the same direction as the wave.

(b)(ii)
- correct formula or substitution: \(\lambda = \frac{v}{f}\) OR \(\frac{340}{850}\) [1];
- \(0.40\text{ m}\) / \(0.4\text{ m}\) (correct numerical value and unit required for 2nd mark) [1];
ALLOW: \(40\text{ cm}\) if unit is clearly stated.

(a) Use the parallel resistance formula:
\(\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} = \frac{1}{30} + \frac{1}{60} = \frac{2}{60} + \frac{1}{60} = \frac{3}{60} = \frac{1}{20}\)
\(R_p = 20\,\Omega\)

(b) The total resistance is the series sum of the parallel combination and \(R_3\):
\(R_{\text{total}} = R_p + R_3 = 20 + 40 = 60\,\Omega\)

(c)(i) Total current from the supply (which passes through the ammeter in series):
\(I = \frac{V}{R_{\text{total}}} = \frac{12.0}{60} = 0.20\text{ A}\)

(c)(ii) Power dissipated in \(R_3\):
\(P = I^2 R_3 = (0.20)^2 \times 40 = 0.040 \times 40 = 1.6\text{ W}\)
(Alternatively, \(V_3 = IR_3 = 0.20 \times 40 = 8.0\text{ V}\), then \(P = IV = 0.20 \times 8.0 = 1.6\text{ W}\))

(d) The potential difference across the parallel combination decreases.
Explanation: The circuit acts as a potential divider where the supply voltage is shared in proportion to resistance. Increasing the resistance of \(R_3\) causes \(R_3\) to take a larger share of the total \(12.0\text{ V}\) supply, leaving a smaller share for the parallel combination (or total current decreases, so \(V_p = I \times R_p\) decreases).

(a) \(\frac{1}{R_p} = \frac{1}{30} + \frac{1}{60}\) OR \(R_p = \frac{30 \times 60}{30 + 60}\) [1];
\(20\,\Omega\) [1];

(b) \(60\,\Omega\) (ALLOW ECF from (a): \(R_p + 40\)) [1];

(c)(i) \(I = \frac{V}{R}\) OR \(\frac{12.0}{60}\) [1];
\(0.20\text{ A}\) (ALLOW ECF from (b)) [1];

(c)(ii) \(P = I^2 R\) OR \(P = IV\) with \(V = 8.0\text{ V}\) [1];
\(1.6\text{ W}\) (ALLOW ECF from (c)(i)) [1];

(d) (potential difference) decreases [1];
\(R_3\) takes a greater share/fraction of the supply voltage OR total current in the circuit decreases (and \(V = IR\)) [1]