Required Practicals

Every required practical can appear in your exams in four ways — learn each practical with all four in mind:

• Describe — write or order the method steps, name the apparatus, state the measurements taken.
• Explain — justify a step (why excess base? why a polystyrene cup? why pencil, not pen?). This is where most marks live.
• Apply — process results: plot graphs, calculate gradients, R_f values or means, spot anomalies.
• Evaluate — identify errors, suggest improvements, and comment on accuracy, repeatability and validity.

Paper 1 examines RP1–RP4 (topics C1–C5); Paper 2 examines RP5–RP8 (topics C6–C10). RP2 and RP7 are Triple Chemistry only.

Copper sulfate is only the textbook example. The exam can name any soluble salt — zinc chloride, magnesium sulfate, copper nitrate — and expect you to choose reactants that actually make it. Writing “add a base to an acid” is the classic dropped mark: the examiner wants the specific acid and the specific base, linked to the salt.

• The second word of the salt’s name tells you the acid: chloride → hydrochloric acid, sulfate → sulfuric acid, nitrate → nitric acid.
• The first word tells you the metal the insoluble base must contain — an oxide, hydroxide or carbonate of that metal.

So for zinc chloride: warm hydrochloric acid, then add zinc oxide (or zinc carbonate) to excess — same method as above, different reactants. Remember a carbonate also fizzes off carbon dioxide as it neutralises the acid.

It depends on whether it dissolves. An insoluble hydroxide behaves just like an oxide or carbonate — add it to excess, filter, crystallise. But a soluble hydroxide such as sodium hydroxide is an alkali: it’s already dissolved in solution. The whole method relies on being able to add an excess of solid and then filter the excess out — you can’t filter a dissolved excess, so any extra alkali stays mixed in with the salt and the sample isn’t pure.

AQA’s wording is precise: a Bunsen burner to heat the dilute acid, but a water bath or electric heater to evaporate the solution. A water bath can never exceed 100 °C, so the heating is gentle and even — the solution doesn’t spit, and the salt isn’t overheated and decomposed. “Why is a water bath used?” is a regular 1–2 mark question.

Required practical questions often end with: “Give one safety precaution the student should take. Explain why.” “Be careful” and “wear a lab coat” score nothing. Name the hazard, give the precaution that targets it, and say what harm it prevents: “wear safety goggles because the dilute acid is an irritant and could damage the eyes”. The dilute acids at GCSE are usually classed as irritant rather than corrosive — and the heating stages carry their own hazards: stop heating before the solution spits, and let the evaporating basin cool before moving it.

In AQA’s own method the gas volumes are usually too small to test with splints. Chlorine is identified directly (damp blue litmus bleaches white); a gas at the anode that does not bleach litmus is taken to be oxygen; any gas at the cathode is taken to be hydrogen. In an exam answer, still quote the full gas tests — pop, relight, bleach — when asked how to identify a gas.

The cup and lid are the key pieces of kit: polystyrene is a good insulator, so it reduces the energy transferred between the mixture and the surroundings — making the temperature change more accurate. For an exothermic reaction the insulation slows energy being lost; for an endothermic one it slows energy being gained — so for an endothermic reaction, write that it “prevents energy being gained”, not that it “prevents heat loss”.

And when a question asks why the cup makes the result more accurate, the answer must be comparative: “less energy is transferred to the surroundings than with a glass beaker”. Absolute claims — “no energy is lost” — are wrong (some always is) and score nothing.

A classic graph plots the temperature change against the mass or volume of one reactant you add to a fixed quantity of the other. Up to the point where the reactants are in their reacting proportions, the temperature change increases as you add more. What happens after that depends on what you added — and explaining it is where the marks are:

• add a solid and the line levels off (a plateau): the other reactant is now used up, so the extra solid just sits there unreacted and makes no further difference;
• add a solution and the line peaks then dips: the excess cold solution cools and dilutes the mixture, so the reading falls back.

Don’t be thrown if a question draws the plateau sloping gently downward rather than dead flat — once the reaction is over, the warm solution slowly cools toward room temperature, so the reading drifts down. The mark is still for spotting that the reaction stops adding heat once the limiting reactant is used up.

Don’t mix that up with a temperature–time graph of a single run, where the across-the-bottom axis is time rather than the mass or volume you added. Here the temperature shoots up to a maximum as the reaction happens, then slowly falls back: once the reaction has finished no more energy is released, so the warm mixture loses heat to the cooler surroundings.

The mean rate over a period is volume ÷ time — but the rate at one moment is the gradient of the tangent to the curve at that time. Draw the tangent touching the curve at the point, make the gradient triangle large (small triangles magnify drawing errors), show your working on the graph, and quote the answer with its unit (e.g. cm³/s). Questions sometimes ask for the answer in standard form — e.g. 0.0025 = 2.5 × 10⁻³.

• Same depth of solution every time — you look down through the liquid, so a deeper mixture hides the cross sooner. Topping up with water when diluting keeps the total volume, and so the depth, the same.
• Measure volumes with a measuring cylinder, not a beaker — its finer scale (better resolution) makes the “same total volume” genuinely the same.
• State the observation, not an inference — timing stops when “the cross is no longer visible”. “The reaction finished” is an inference, and a false one: the reaction is still going; there’s just enough sulfur to block your view.
• The end-point judgement is the weakness — improve it with the same observer every time, or remove the judgement entirely with a light sensor and data logger.
• Safety — the reaction makes sulfur dioxide, a toxic gas: keep the room well ventilated.

• Pure / impure — categorical, not relative. A single spot means the substance is pure; more than one spot means impure. Writing “purer” or “less pure” misses the mark — tie the word to the number of spots.
• Why a dye travels further — the marks want both halves of the balance: it is more soluble in the solvent and has a weaker attraction to the paper. Solubility alone is usually not enough.
• What R_f really says — for a given substance, solvent and paper, the spot and the solvent travel in a constant proportion: that ratio is the R_f, which is why it identifies the substance regardless of how long the paper runs.

Carbonate ions would also form a precipitate with silver nitrate or barium chloride, giving a false positive. Adding acid first reacts away any carbonate (it fizzes off as CO₂) so it can’t interfere. The catch is choosing an acid that doesn’t add the very ion you’re testing for: nitric acid for the halide test (hydrochloric would add chloride ions), and hydrochloric acid for the sulfate test (sulfuric would add sulfate ions).

Six-mark plan questions here are marked on whether the method would actually deliver an answer for the sample in the question. Students who list correct tests in the abstract — without saying they would carry them out on the sample, or in a workable order — can score nothing despite knowing every test. Structure the plan: take (a solution of) the sample → flame test for the cation → one anion test at a time, naming the reagent and the expected result for a positive.

AQA’s student sheet tests the salt water chemically, before and after distilling — a neat link to the ion tests of RP7: a flame test (yellow = sodium ions) and dilute nitric acid + silver nitrate (white precipitate = chloride ions). The salt water gives both positives; the distillate — collected through a delivery tube into a test tube standing in ice water to condense the steam — gives neither. No yellow flame, no precipitate: the ions have been left behind, so the water is purified. A likely exam angle: “How could the student show the distilled water no longer contains chloride ions?”

A real exam question typically gives a table of voltages for pairs of metals (metal 1 down the side, metal 2 across the top) and defines a sign convention in the stem — for example: “if metal 2 is more reactive than metal 1, the voltage is positive; if metal 1 is more reactive, it is negative.” Read that convention from the question every time rather than memorising one — the examiner sets it, and it can differ between papers. Then three reads unlock the whole table:

• Same metal → 0 V — no reactivity difference, no voltage. These entries run down the table’s diagonal.
• The size of the voltage ranks the gap in reactivity: the pair furthest apart in the series gives the largest reading.
• The sign tells you which of the two metals is the more reactive one — apply the stem’s convention.

Here’s the format with example results — metal 1 down the side, metal 2 across the top, and only half the grid measured (swapping the metals just flips the sign, so the other half is redundant):

Reading it with the convention above: every copper-column voltage is negative, so copper is less reactive than everything — and magnesium’s row has the largest readings, so magnesium is furthest from the others. The full order falls out: magnesium > zinc > iron > copper.

And the classic follow-up — predict the voltage for the cell that wasn’t measured (zinc/iron): from copper’s column, zinc beats copper by 1.1 V and iron beats copper by 0.7 V, so the zinc–iron gap is about 0.4 V. Metal 2 (iron) is less reactive than metal 1 (zinc), so the convention makes it −0.4 V. Both the size and the sign earn marks. Questions often include unfamiliar metals (chromium, tin) precisely so you can’t rely on a memorised series — the data is the series.

• Never write “amount”. Say exactly what you are changing, measuring or keeping the same — the mass, volume, concentration or number of moles. “The same volume of acid”, not “the same amount of acid”.
• Safety is a matched pair. Name the hazard, give the precaution that targets it, and say what harm it prevents.
• Observation, then inference. Write down what you would see (“the cross is no longer visible”, “a white precipitate forms”), and keep it separate from what it means.
• Right + wrong = wrong. A correct point contradicted elsewhere in the same answer is cancelled — mark schemes do not let a wrong extra ride along with a right one. Don’t scatter alternatives to “cover all eventualities”; commit to the correct one.
• Six-markers: task → sequence → judgement. Work out exactly what the question needs you to produce, put the steps in an order that would actually work, and — if the command implies a decision — state the judgement. Length adds nothing; structure is what moves an answer up a level.