AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 7

8 marks · Standard Demand difficulty · Extended Answer

Identify four white oxide solids from their solubility and pH results, then calculate the pH after diluting one acidic solution.

Practise this question

Question

The question page contains two linked parts about four white solids labelled A, B, C and D. In part 07.1, a scientist adds each solid to distilled water, stirs, and then measures pH; Table 2 shows A gives a colourless solution with no solid and pH 14, B gives a colourless solution with no solid and pH 3, C gives a colourless solution with solid remains and pH 9, and D gives a colourless liquid with solid remains and pH 7. A second table lists the possible identities and solubilities in grams per 100 cubic centimetres of water: magnesium oxide 0.01, phosphorus oxide 52, silicon dioxide 0, and sodium oxide 109; students must identify A to D and explain their answers for 6 marks. In part 07.2, 10 cubic centimetres of solution B at pH 3 is diluted with distilled water to a final volume of 1000 cubic centimetres, and Table 4 asks students to calculate the new pH X for 2 marks.
Question text

07 A scientist does two tests on four white solids. The solids are labelled A, B, C and D.

Test 1 Adds the sample of the solid to distilled water and stirs.

Test 2 Measures the pH of the solution after Test 1

Table 2 shows the results.

Table 2

Solid Appearance after stirring pH

A colourless solution, no solid 14

B colourless solution, no solid 3

C colourless solution, solid remains 9

D colourless liquid, solid remains 7

These four solids are:

• magnesium oxide

• phosphorus oxide

• silicon dioxide

• sodium oxide.

Table 3 shows the solubility of these four solids in water.

Table 3

Solid Solubility in grams per 100 cm3 of water

Magnesium oxide 0.01

Phosphorus oxide 52

Silicon dioxide 0

Sodium oxide 17 109

07.1 Identify the solids A, B, C and D.

Explain your answers.

[6 marks]

10 cm3 of solution B is added to a beaker.

07.2

Distilled water is added to the beaker until the final volume in the beaker is 1000 cm3

The pH of the solution is measured before and after distilled water is added.

Table 4 shows the results.

Table 4

Volume of solution in beaker pH of solution B

*17* 10 cm3 3

1000 cm3 X

Calculate the value of X.

[2 marks]

X =

Mark scheme

Show the mark scheme The mark scheme awards question 07.1 using levels: Level 1 for simple identified points, Level 2 for some logical linking, and Level 3 for detailed logically linked reasoning, with 5 to 6 marks requiring correct identification of all solids. Indicative content states A is sodium oxide because it is the most alkaline and fully dissolves, B is phosphorus oxide because it is acidic and fully dissolves, C is magnesium oxide because it gives an alkaline solution with some solid remaining, and D is silicon dioxide because it is neutral with undissolved solid; the scheme also credits reasoning based on solubility and the difference between colourless solution and colourless liquid. For question 07.2, the mark scheme gives one mark for recognising a dilution by a factor of 100 and one mark for the final answer pH 5, with equivalent stepwise reasoning using pH changes by 1 for each tenfold dilution also accepted.

AO /

Question Answers Mark

Spec. Ref.

Level 3: Relevant points (reasons / causes) are identified, given in 5–6 AO3

07.1

detail and logically linked to form a clear account.

Level 2: Relevant points (reasons / causes) are identified, and 3–4 AO3

there are attempts at logical linking. The resulting account is not

fully clear.

Level 1: Points are identified and stated simply, but their relevance 1–2 AO1

is not clear and there is no attempt at logical linking. AO2

No relevant content 0

Indicative content: 5.1.2.3

5.4.2.4

A is sodium oxide

B is phosphorus oxide

C is magnesium oxide

D is silicon dioxide

linked statements:

A is sodium oxide

because it has highest pH or pH = 14 or is a strong alkali

B is phosphorus oxide

because it has lowest pH or pH = 3 or is an acid

C is magnesium oxide

because it has 2nd highest pH or pH = 9 or is a (weak) alkali

D is silicon dioxide

because it is neutral or pH = 7

or

A and B are sodium oxide or phosphorus oxide

because both soluble or no solid remains

C is magnesium oxide

because it will be the colourless solution with solid remaining

D is silicon dioxide

because it will be the colourless liquid with solid remaining

for level 3 the solids must be correctly identified

AO /

Question Answers Extra information Mark

Spec. Ref.

07.2 an answer of (pH=) 5 gains 2

marks 16

dilution by a factor of 100 allow pH changes by 1 when 1 AO1

solution is diluted by factor of 10 5.4.2.5

or

allow pH changes by 2

(pH=) 5 1 AO3

5.4.2.5

Total 8

How to answer it

AQA GCSE Combined Science: Trilogy · Chemistry

Identifying unknown oxides and using dilution to find pH

What this question tests

This question checks whether you can use solubility, appearance after stirring, and pH to identify substances. You also need to understand that diluting an acid or alkali changes the pH, and in a simple dilution question you can use the change in concentration to work out the new pH.

Marks are awarded for correct identification, linked reasons, and using the dilution rule correctly.

Question 07.1 — Identify solids A, B, C and D

6 marks: full marks need correct identification + clear reasons

✅ Correct answers

  • A = sodium oxide
  • B = phosphorus oxide
  • C = magnesium oxide
  • D = silicon dioxide

Top-band answers do more than name the solids: they link the result to pH and solubility.

💡 Key knowledge

  • Sodium oxide is a strong alkali when dissolved in water, so it gives a high pH.
  • Phosphorus oxide is an acidic oxide, so it gives a low pH.
  • Magnesium oxide is only slightly soluble, so some solid can remain and the solution is alkaline.
  • Silicon dioxide is insoluble, so solid remains and the water stays close to neutral.

🧠 Exam technique

  • Use all the evidence: appearance after stirring + pH.
  • Pick out the most acidic, most alkaline, and neutral results first.
  • Then match the solids to the solubility table if needed.
  • For the highest level, your explanation should be a clear chain of reasoning, not just a list of facts.

❌ Common errors

  • Thinking all oxides are neutral — they are not.
  • Mixing up soluble and insoluble.
  • Saying only “A is sodium oxide” without explaining why.
  • Ignoring the appearance after stirring: no solid remains is important evidence.

How to build the answer step by step

✅ A

A = sodium oxide because it gives the highest pH of 14, showing a strong alkali. It also dissolves completely, so no solid remains.

Awarded when the answer links pH 14 to sodium oxide.

✅ B

B = phosphorus oxide because it gives the lowest pH of 3, so it is acidic. It dissolves completely, so no solid remains.

This is the clearest acidic result in the table.

✅ C

C = magnesium oxide because the solution is alkaline with pH 9, but some solid remains. This fits a solid that is only slightly soluble.

✅ D

D = silicon dioxide because the pH is 7, which is neutral, and solid remains. This matches an insoluble solid.

📐 Why the solubility table matters

  • Sodium oxide: very soluble, so no solid remains and the solution becomes strongly alkaline.
  • Phosphorus oxide: soluble enough to give a solution and an acidic pH.
  • Magnesium oxide: only 0.01 g per 100 cm³, so a little dissolves but some solid remains.
  • Silicon dioxide: 0 g per 100 cm³, so it does not dissolve and the water stays neutral.

The examiner rewarded students who used both pH evidence and solubility evidence in a linked explanation.

❌ What lost marks here

  • Giving the right names but no explanation.
  • Matching solids only by pH and forgetting the solid remaining information.
  • Claiming magnesium oxide is highly soluble — it is only slightly soluble.
  • Calling silicon dioxide alkaline because it is an oxide — this one is not behaving like a base in water.

Question 07.2 — Calculate the pH after dilution

2 marks: one for dilution by a factor of 100, one for the new pH

✅ Correct answer

X = 5

The solution is diluted by a factor of 100, so the pH increases by 2 from 3 to 5.

📐 Step-by-step calculation

  1. Start with 10 cm³ of solution B at pH 3.
  2. Final volume = 1000 cm³.
  3. Calculate the dilution factor: 1000 ÷ 10 = 100 .
  4. A 100× dilution means the concentration of acid becomes 100 times smaller.
  5. For pH, each 10× dilution changes pH by 1, so a 100× dilution changes pH by 2.
  6. So the new pH is 3 + 2 = 5 .

🧠 Exam technique

  • Always compare the final volume with the starting volume.
  • Use the word dilution in your answer — the mark scheme accepts this.
  • If you know the pH rule: 10× dilution → pH changes by 1.
  • Keep the answer simple and give the final value clearly: X = 5.

❌ Common errors

  • Adding 100 to the pH instead of using the dilution rule.
  • Saying the pH becomes 7 just because water was added.
  • Forgetting the starting pH is 3.
  • Using the wrong direction: dilution of an acid makes pH increase, not decrease.

💡 Extra knowledge: what AQA was looking for

The mark scheme accepts either:

  • “dilution by a factor of 100” and pH = 5, or
  • recognising that each 10× dilution changes pH by 1, so 100× changes it by 2.

This means you can still gain credit if your working is brief, as long as the idea is correct.

Model 6-mark response for 07.1

A is sodium oxide because it gives the highest pH, 14, so it is a strong alkali and it dissolves fully with no solid left. B is phosphorus oxide because it gives the lowest pH, 3, so it is acidic and also dissolves fully. C is magnesium oxide because the solution is alkaline with pH 9 but some solid remains, showing it is only slightly soluble. D is silicon dioxide because the pH is 7, so it is neutral, and solid remains because it is insoluble.

This style would reach the top level because it identifies all four solids correctly and links each one to the correct evidence.

Quick memory recap

💡 Match the evidence

  • pH 14 → sodium oxide
  • pH 3 → phosphorus oxide
  • pH 9 + solid remains → magnesium oxide
  • pH 7 + solid remains → silicon dioxide

🧠 How to get marks fast

  • Name the solid.
  • Give the pH or solubility reason.
  • Use precise science words: acidic, alkaline, neutral, soluble, insoluble.

Topics

Chemistry · C4: Chemical Changes

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 1 (Higher), 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.