13.8 - Planning an Investigation or Describing a Method

What a planning question looks like

Question 4 is always worth 6 marks. It takes one of two forms:

1. Form 1 - Plan an investigation you have not carried out (e.g. "Plan an investigation to find which of lime juice and lemon juice contains the more concentrated aqueous citric acid.")
2. Form 2 - Describe how to obtain or purify a substance (e.g. "Describe how to make a pure, dry sample of solid lead(II) chloride.")

How Q4 is marked

• Both forms are marked "any 6 from" a list of around seven method points.
• There's no single "right" experiment. The examiner checks whether the required method points are present, not whether your answer matches a single model method.

Safety never scores here

• A safety precaution earns no marks in Q4, unlike Q1 where safety points are credited.
• Spend your points on the method, not on safety.

The "fair test" exception

• When asked to plan an investigation into the order of reactivity of metals, the mark scheme credits "fair test" as its own heading for naming what must be kept constant (e.g. the same volume and concentration of acid for each metal tested).
• Everywhere else on the paper, keeping variables constant is credited without needing that label.

A typical question

Limes and lemons are citrus fruits which contain aqueous citric acid in their juice. Plan an investigation to find which of lime juice and lemon juice contains the more concentrated aqueous citric acid.

Planning an investigation

• A high-scoring plan covers the same six ingredients every time.
• The 6 marks are spread across these ingredients, so touching on all six briefly scores better than describing the method in detail while omitting quantities or processing.

The six ingredients of a plan

Worked through for comparing lime and lemon juice by titrating each against the same alkali:

The plan written out

"Measure 25 cm³ of lime juice into a conical flask and add five drops of methyl orange. Add aqueous sodium hydroxide from a burette, swirling the flask, until the indicator just changes colour. Record the volume, repeat, and calculate a mean. Repeat for the lemon juice, keeping the volume and concentration of sodium hydroxide the same. The juice needing the larger mean volume contains the more concentrated citric acid."

Describing how to obtain or purify a substance

• This question gives you starting materials and asks for the steps needed to produce a pure, dry sample of a named substance.
• Marking is "any 6 from" a list of points, where the points are steps in a sequence.

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Worked example - Making pure, dry lead(II) chloride

From solid sodium chloride and solid lead(II) nitrate:

2NaCl(aq) + Pb(NO₃)₂(aq) → 2NaNO₃(aq) + PbCl₂(s)

1. Dissolve the solid sodium chloride in distilled water in a beaker.
2. Dissolve the solid lead(II) nitrate in distilled water in a second beaker.
3. Pour the two solutions together into a suitable container.
4. Stir the mixture so the reactants mix fully and a precipitate of lead(II) chloride forms.
5. Filter the mixture to collect the lead(II) chloride as the residue on the filter paper.
6. Wash the residue with a little distilled water, then dry it (e.g. between sheets of filter paper or in a warm oven) to obtain a pure, dry sample.

Together these steps cover dissolving the reactants, combining them, separating the precipitate, and washing and drying the product.

Common exam questions

Question 1

Question 4 is marked "any 6 from" a list of about seven points. Why does this matter for how you write your answer?

Because there are more ways to score than marks available, so you do not need to include every possible point; six clearly written points that each address the mark scheme are enough for full marks.

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Question 2

Should a plan include a safety precaution?

No. A safety point earns nothing in Q4, in any of the mark schemes checked; the six marks are for the method itself, so a line spent on safety is a line not spent on something that could score.

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Question 3

A student writes a full paragraph on the titration method but never says how the results are used to answer the question. What mark is lost?

The processing mark - the plan needs a final sentence saying, for example, that the juice needing the larger volume of alkali is the more concentrated one, otherwise the results are never actually used to reach a conclusion.

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Question 4

Why does "add some sodium hydroxide" score less than "add 25 cm³ of sodium hydroxide from a burette"?

Because the first gives no specific quantity or apparatus, while the second states both, and only a stated amount and named apparatus score the quantities and apparatus marks.

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Question 5

A plan asks you to compare the reactivity of three metals with dilute hydrochloric acid. What is different about how this particular plan is marked?

It is one of the few plans where "fair test" is itself a credited mark heading, worth several of the six marks, for stating that the same volume, concentration, and temperature of acid, and the same mass and form of each metal, are used for every metal tested.

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Question 6

A student describes making a pure, dry sample of a salt but stops after filtering the precipitate. What is missing?

Washing the residue and drying it. The question asks for a pure, dry sample, so the method needs to say how the solid is washed to remove other dissolved substances and then dried, not just collected on the filter paper.

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Question 7

Why does a plan that briefly covers all six ingredients usually score better than one that writes at length about the method alone?

Because the six marks are spread one across each ingredient - apparatus, quantities, what is kept constant, what is measured, repeats, and processing the results - so a plan that touches all six, even briefly, can reach full marks, while a long method description that omits the other five ingredients cannot.