Practical skills assessed in written papers (PS 1.1-4.1, AT a-l)

Welcome to AQA A-Level Practical Chemistry

Practical chemistry is the beating heart of the chemical sciences. In AQA Chemistry (7405), practical work is not just an extra classroom activity—it makes up at least 15% of the total marks across your written exam papers (Paper 1, Paper 2, and Paper 3). In fact, Paper 3 includes 40 marks dedicated directly to practical techniques and data analysis!

You also work towards the Practical Endorsement (CPAC), a teacher-assessed competency reported directly on your certificate as Pass or Not Reported.

Don't worry if experimental questions seem intimidating at first. Once you understand the underlying principles of the apparatus, how to eliminate experimental errors, and the exact steps for each required practical activity, you will find these questions among the most predictable and straightforward marks in your exams.

Core Apparatus and Techniques (AT a to AT l)

The AQA specification outlines twelve core techniques that every chemist must master:

AT a: Measuring mass, time, temperature, and volumes of liquids and gases using balances, stopwatches, thermometers, measuring cylinders, pipettes, burettes, and gas syringes.
AT b: Safe heating using water baths, electric heating mantles, or sand baths.
AT c: Measuring \( \text{pH} \) using indicator papers, calibrated \( \text{pH} \) meters, or \( \text{pH} \) probes with data loggers.
AT d: Standard laboratory setups: titrations, distillation, heating under reflux, qualitative test-tube reactions, gravity filtration (fluted filter paper), and vacuum filtration (Buchner funnel).
AT e: Preparing standard volumetric solutions using volumetric flasks.
AT f: Selecting and using acid–base indicators (e.g., phenolphthalein and methyl orange).
AT g: Purifying solids via recrystallisation and purifying liquids using separating funnels and solvent extraction.
AT h: Determining melting points and boiling points to verify purity.
AT i: Chromatographic separation using Thin Layer Chromatography (TLC) or paper chromatography.
AT j: Setting up electrochemical cells and measuring \( E_{\text{cell}} \) using a high-resistance voltmeter.
AT k: Risk assessment, chemical hazard management, and safe disposal.
AT l: Monitoring reaction rates via initial rate methods (clock reactions) and continuous monitoring (gas volume, mass loss, colorimetry).

The 12 Required Practical Activities (RP1 – RP12)

RP1: Making a Standard Solution and Acid–Base Titration

Purpose: Prepare a known concentration of a primary standard and use it to find the concentration of an unknown acid or base.

Step-by-Step Method for Making a Standard Solution:

1. Weigh the solid sample in a weighing boat on a balance.
2. Tip the solid into a clean beaker and re-weigh the empty boat (weighing by difference to determine the exact mass transferred).
3. Add deionised water to the beaker and stir with a glass rod until the solid completely dissolves.
4. Transfer the solution into a volumetric flask using a funnel. Rinse the beaker, glass rod, and funnel with deionised water and pour the washings into the flask.
5. Add deionised water until the bottom of the meniscus sits exactly on the calibration line (use a dropping pipette for the last few drops at eye level).
6. Invert the stoppered flask at least 10 times to ensure thorough mixing.

Carrying out the Titration:

1. Rinse the volumetric pipette with the solution it will measure, then transfer \( 25.0\text{ cm}^3 \) into a conical flask.
2. Rinse the burette with the titrant, fill it, and ensure the space below the tap (the jet) is filled with liquid and free of air bubbles.
3. Add 2 to 3 drops of indicator (e.g., phenolphthalein: pink in alkali, colourless in acid; or methyl orange: yellow in alkali, red in acid).
4. Place the conical flask on a white tile to observe the colour change clearly.
5. Titrate with swirling, adding dropwise near the end point until the indicator changes colour permanently.
6. Repeat until you achieve concordant titres (titres within \( \pm 0.10\text{ cm}^3 \) of each other).

RP2: Measurement of an Enthalpy Change (\( \Delta H \))

Purpose: Determine the enthalpy change of a reaction in solution using calorimetry.

The Cooling Curve Technique:
When reactions are slow or lose heat rapidly to the surroundings, a standard maximum temperature reading underestimates the true temperature change. Chemists use a cooling curve to correct for heat loss:

1. Place a known volume of reactant into an expanded polystyrene cup (a good thermal insulator).
2. Record its temperature every minute for 3 minutes to establish a steady baseline.
3. At minute 4, add the second reactant and stir, but do not record the temperature at minute 4.
4. Resume recording temperature at minute 5 and continue every minute up to 10–15 minutes.
5. Plot temperature against time. Extrapolate the pre-mixing line forward to minute 4 and the cooling curve back to minute 4. The difference between these two extrapolated lines at minute 4 represents the theoretical temperature change (\( \Delta T \)) with zero heat loss.

Use \( q = mc\Delta T \) (where \( m \) is the mass of solution, usually assuming a density of \( 1.0\text{ g cm}^{-3} \), and \( c = 4.18\text{ J g}^{-1}\text{ K}^{-1} \)), followed by \( \Delta H = -\frac{q}{n \times 1000} \) in \( \text{kJ mol}^{-1} \).

RP3: Investigating Reaction Rate with Temperature

Purpose: Study how temperature affects the rate of the reaction between sodium thiosulfate and hydrochloric acid: \( \text{Na}_2\text{S}_2\text{O}_3\text{(aq)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{SO}_2\text{(aq)} + \text{S(s)} + \text{H}_2\text{O(l)} \).

Method: Measure known volumes of reactants, place the reaction flask over a printed black cross, mix the solutions at a known temperature, and time how long it takes for the precipitate of solid sulfur to obscure the cross from view.
Data Analysis: Rate is proportional to \( \frac{1}{t} \). Plotting \( \ln(\text{rate}) \) or \( \ln\left(\frac{1}{t}\right) \) against \( \frac{1}{T} \) (where \( T \) is in Kelvin) yields an Arrhenius plot with a gradient of \( -\frac{E_a}{R} \).

RP4: Qualitative Tests for Inorganic Ions

Cation Tests:

Group 2 Cations (\( \text{Mg}^{2+}, \text{Ca}^{2+}, \text{Sr}^{2+}, \text{Ba}^{2+} \)):
- With aqueous \( \text{NaOH} \): \( \text{Mg}^{2+} \) forms a thick white precipitate (\( \text{Mg(OH)}_2 \)); \( \text{Ca}^{2+} \) forms a faint white precipitate; \( \text{Sr}^{2+} \) and \( \text{Ba}^{2+} \) show no precipitate (hydroxide solubility increases down Group 2).
- With aqueous \( \text{H}_2\text{SO}_4 \): \( \text{Ba}^{2+} \) forms a thick white precipitate (\( \text{BaSO}_4 \)); \( \text{Sr}^{2+} \) forms a white precipitate; \( \text{Ca}^{2+} \) forms a faint white precipitate; \( \text{Mg}^{2+} \) shows no precipitate (sulfate solubility decreases down Group 2).

Ammonium Ion (\( \text{NH}_4^+ \)):
Warm gently with dilute \( \text{NaOH(aq)} \). Pungent ammonia gas (\( \text{NH}_3 \)) is released, turning damp red litmus paper blue.

Anion Tests:

Halide Ions (\( \text{Cl}^-, \text{Br}^-, \text{I}^- \)):
1. Add dilute nitric acid (\( \text{HNO}_3 \)) to remove interfering carbonate or hydroxide ions.
2. Add aqueous silver nitrate (\( \text{AgNO}_3 \)):
- \( \text{Cl}^- \): White precipitate (\( \text{AgCl} \)), dissolves in dilute \( \text{NH}_3\text{(aq)} \).
- \( \text{Br}^- \): Cream precipitate (\( \text{AgBr} \)), insoluble in dilute \( \text{NH}_3\text{(aq)} \), dissolves in concentrated \( \text{NH}_3\text{(aq)} \).
- \( \text{I}^- \): Yellow precipitate (\( \text{AgI} \)), insoluble in both dilute and concentrated \( \text{NH}_3\text{(aq)} \).

Sulfate Ion (\( \text{SO}_4^{2-} \)):
Add dilute hydrochloric acid (\( \text{HCl} \)) followed by aqueous barium chloride (\( \text{BaCl}_2 \)). A dense white precipitate of \( \text{BaSO}_4 \) forms.

Carbonate Ion (\( \text{CO}_3^{2-} \)):
Add dilute acid (e.g., \( \text{HCl} \)). Effervescence occurs; the gas produced turns limewater cloudy (\( \text{CO}_2 \)).

Hydroxide Ion (\( \text{OH}^- \)):
Turns red litmus paper blue, or produces an alkaline reading with universal indicator / \( \text{pH} \) probe.

RP5: Distillation of an Organic Product

Purpose: Synthesise and isolate a volatile organic product (e.g., oxidising a primary alcohol like ethanol to ethanal using acidified potassium dichromate(\(\text{VI}\))).

Apparatus Setup:
- Pear-shaped or round-bottom flask containing the reaction mixture and anti-bumping granules (which ensure smooth boiling and prevent large bubbles).
- Still head with a thermometer.
- Liebig condenser angled downwards with cooling water flowing in at the bottom and out at the top.
- Collection vessel placed in an ice bath to prevent evaporation of the volatile product.

RP6: Tests for Organic Functional Groups

Alkene (\( \text{C}=\text{C} \)): Shake with bromine water (\( \text{Br}_2\text{(aq)} \)). Solution changes from orange/brown to colourless.
Alcohol (\(-\text{OH}\)): Warm with acidified potassium dichromate(\(\text{VI}\)) (\( \text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4 \)). Primary and secondary alcohols turn the solution from orange to green (\( \text{Cr}^{3+} \)). Tertiary alcohols show no colour change.
Aldehyde (\(-\text{CHO}\)):
- Tollens' reagent: Warm gently; a silver mirror forms on the inner wall of the test tube (\( \text{Ag}^+ \) reduced to \( \text{Ag(s)} \)).
- Fehling's solution: Warm gently; blue solution forms a brick-red precipitate of \( \text{Cu}_2\text{O} \).
Carboxylic Acid (\(-\text{COOH}\)): Add solid sodium hydrogencarbonate (\( \text{NaHCO}_3 \)) or sodium carbonate (\( \text{Na}_2\text{CO}_3 \)). Effervescence of \( \text{CO}_2 \) gas occurs, which turns limewater cloudy.

RP7: Measuring Rates of Reaction

RP7a (Initial Rate / Clock Reaction):
In the iodine clock reaction (e.g., hydrogen peroxide reacting with iodide ions in the presence of starch and a fixed small amount of sodium thiosulfate), the time \( t \) taken for the dark blue-black starch-iodine complex to suddenly appear is measured. Since \( \text{Rate} \propto \frac{1}{t} \), varying reactant concentrations allows the deduction of individual reaction orders.

RP7b (Continuous Monitoring):
A reaction that produces a gas (such as magnesium reacting with hydrochloric acid) is carried out in a closed system connected to a gas syringe. The total volume of gas collected is recorded at regular time intervals (e.g., every 10 seconds). A volume-time graph is plotted, and the gradient of a tangent drawn at \( t = 0 \) gives the initial rate.

RP8: Measuring the EMF of an Electrochemical Cell

Purpose: Measure cell potential (\( E_{\text{cell}} \)) for metal/metal-ion systems.

Setup:
1. Clean metal strips (electrodes) with emery paper to remove oxide layers.
2. Dip each metal electrode into a \( 1.00\text{ mol dm}^{-3} \) solution of its respective metal ions.
3. Connect the two half-cells using a salt bridge (filter paper soaked in saturated \( \text{KNO}_3 \) or \( \text{KCl} \)) to allow ions to flow and maintain electrical neutrality without mixing solutions.
4. Connect the electrodes to a high-resistance voltmeter (which ensures virtually no current flows, measuring the true maximum EMF).

RP9: Investigating pH Changes during Titrations

Purpose: Generate a titration curve by continuously tracking \( \text{pH} \) using a calibrated \( \text{pH} \) probe.

Calibration: Calibrate the probe using commercial buffer solutions of known \( \text{pH} \) (e.g., \( \text{pH } 4.00, 7.00, 10.00 \)) and plot a calibration curve to correct for probe inaccuracies.
Method: Add titrant in \( 1.0\text{ cm}^3 \) increments from a burette, stirring thoroughly. As you approach the expected end point, add the titrant in small \( 0.10\text{ cm}^3 \) increments to capture the steep vertical inflection on the \( \text{pH} \) curve.

RP10: Preparation and Purification of Organic Compounds

RP10a: Organic Solid (e.g., Aspirin):
- Recrystallisation: Dissolve the impure solid in the minimum volume of hot solvent. Filter hot through fluted filter paper to remove insoluble impurities. Allow the solution to cool slowly; crystals of pure product precipitate while soluble impurities remain in solution. Collect crystals by vacuum filtration using a Buchner funnel and flask. Wash with a small amount of ice-cold solvent and dry.
- Melting Point Determination: Place dry powdered sample in a sealed capillary tube. Heat slowly in a melting point apparatus. A pure solid melts sharply over a narrow range (within \( 1\text{--}2^\circ\text{C} \)) matching the data-book value. Impurities lower the melting point and broaden the melting range.

RP10b: Organic Liquid (e.g., Cyclohexene from Cyclohexanol):
- Reflux & Distillation: Heat the alcohol with concentrated phosphoric/sulfuric acid. Collect the crude distillate.
- Separation: Transfer the distillate into a separating funnel. Add aqueous sodium hydrogencarbonate to neutralise acid impurities (release pressure regularly by opening the tap). Allow layers to separate; run off the aqueous layer.
- Drying: Add an anhydrous inorganic salt (e.g., anhydrous \( \text{CaCl}_2 \) or \( \text{MgSO}_4 \)) to the organic layer until the drying agent remains powdery and the liquid turns clear.
- Final Distillation: Redistil the dried liquid and collect the fraction boiling at the specific boiling point of cyclohexene.

RP11: Identification of Transition-Metal Ions

Test aqueous metal ions with aqueous sodium hydroxide (\( \text{NaOH} \)) and dilute ammonia (\( \text{NH}_3 \)):

Iron(II) (\( [\text{Fe}(\text{H}_2\text{O})_6]^{2+} \)): Forms a green precipitate of \( \text{Fe}(\text{H}_2\text{O})_4(\text{OH})_2 \), which slowly oxidises to a brown precipitate at the surface in air. Insoluble in excess \( \text{NaOH} \) or excess \( \text{NH}_3 \).
Iron(III) (\( [\text{Fe}(\text{H}_2\text{O})_6]^{3+} \)): Forms a brown precipitate of \( \text{Fe}(\text{H}_2\text{O})_3(\text{OH})_3 \). Insoluble in excess \( \text{NaOH} \) or excess \( \text{NH}_3 \).
Copper(II) (\( [\text{Cu}(\text{H}_2\text{O})_6]^{2+} \)): Forms a blue precipitate of \( \text{Cu}(\text{H}_2\text{O})_4(\text{OH})_2 \). Insoluble in excess \( \text{NaOH} \), but dissolves in excess dilute \( \text{NH}_3 \) to form a deep blue solution of \( [\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+} \).
Aluminium(III) (\( [\text{Al}(\text{H}_2\text{O})_6]^{3+} \)): Forms a white precipitate of \( \text{Al}(\text{H}_2\text{O})_3(\text{OH})_3 \). Dissolves in excess \( \text{NaOH} \) to form a colourless solution of \( [\text{Al}(\text{OH})_4]^- \) (amphoteric behavior), but remains insoluble in excess \( \text{NH}_3 \).

RP12: Thin Layer Chromatography (TLC)

Method: Spot a small drop of the sample mixture onto the pencil baseline of a silica TLC plate. Place the plate in a beaker with a shallow layer of solvent (below the baseline) and cover with a lid.
Visualisation: Once the solvent front approaches the top, mark the solvent front with a pencil. Visualise spots under UV light or with a staining agent (e.g., iodine crystals or ninhydrin).
Calculation:
\( R_f = \frac{\text{Distance moved by amino acid/compound}}{\text{Distance moved by solvent front}} \)

Uncertainty, Precision, and Mathematical Conventions

Calculating Percentage Uncertainty

For any single measuring device:

\( \text{Percentage Uncertainty} = \frac{\text{Uncertainty of measurement}}{\text{Quantity measured}} \times 100 \)

Apparatus Uncertainties

Burette Readings: A burette reading has an uncertainty of \( \pm 0.05\text{ cm}^3 \). Because every titre requires two scale readings (initial and final), the total uncertainty for a delivered titre is \( 2 \times 0.05 = \pm 0.10\text{ cm}^3 \).
Balances: If you weigh a solid by difference (two separate mass measurements), the total balance uncertainty is double the resolution of the balance.
Total Apparatus Uncertainty: Calculated by summing the individual percentage uncertainties of all measured values used in the calculation.

Titration Mathematics & Significant Figures

1. Concordance: Only titres within \( 0.10\text{ cm}^3 \) of each other are concordant. Discard non-concordant titres (and the initial rough trial) before calculating the mean titre.
2. Recording Precision: Burette readings must always be recorded to 2 decimal places, with the second decimal place ending in either \( .00 \) or \( .05 \) (e.g., \( 23.45\text{ cm}^3 \) or \( 24.10\text{ cm}^3 \), never \( 23.4\text{ cm}^3 \)).
3. Significant Figures in Answers: Final calculated answers must be quoted to the same number of significant figures as the least accurate experimental measurement provided in the data.

Examiner Watchpoints & Common Student Pitfalls

1. Rinsing Glassware:
- Burette and Pipette: Must be rinsed with the specific solution they will contain. Rinsing with water dilutes the solution, altering the concentration and invalidating your results.
- Conical Flask: Must be rinsed only with deionised water. Rinsing with acid or alkali adds extra, unmeasured moles of reactant.

2. Air Bubble in the Burette Jet:
Always ensure the jet below the tap is filled with liquid before taking the initial reading. If an air bubble is present and dispenses during the titration, it will be recorded as delivered liquid, giving a falsely large titre.

3. Heating Under Reflux vs Distillation:
- Reflux: The condenser is mounted vertically on top of the reaction flask. Never place a stopper in the top of a reflux condenser, as sealed heating leads to dangerous pressure build-up and explosion.
- Distillation: The condenser is mounted at an angle downwards. The thermometer bulb must sit directly adjacent to the side-arm opening leading to the condenser to accurately measure the boiling point of the vapour entering the condenser.

4. Condenser Water Flow:
Water must always enter at the bottom and exit from the top of the condenser jacket. This ensures the entire jacket fills completely with water, preventing air pockets and providing efficient cooling.

Key Takeaway: Practical exam questions assess your understanding of why steps are performed, how systematic and random errors arise, and how to rigorously process experimental data. Master the setups, rinsing rules, and error calculations, and you will secure top marks across all three papers!