AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2019: Question 2

✅ Correct answer

(aq)

Mark point: the substance is in solution, so the state symbol is (aq) .

💡 Key knowledge

• (aq) means dissolved in water.
• Ammonium nitrate solution is a salt in water, so it is not solid, liquid or gas.

🧠 Exam technique

• If the question says solution, the answer is usually (aq) .
• State symbols are easy marks — write the symbol exactly.

❌ Common errors

• Writing (s) because salts are often solids.
• Writing (l) because it is “in liquid”.
• Forgetting the brackets.

✅ Correct answer

HNO₃

Tick HNO₃ only.

💡 Key knowledge

• Nitric acid always has the formula HNO₃.
• It is one of the common strong acids you should know: hydrochloric acid HCl , nitric acid HNO₃ , sulfuric acid H₂SO₄ .

🧠 Exam technique

• Look for the acid name carefully — nitric means nitrate chemistry.
• Only one box should be ticked.

❌ Common errors

• Choosing HCl because it is another acid.
• Choosing H₂SO₄ because it is also a strong acid.
• Choosing NH₄OH — that is not nitric acid.

✅ Correct answers

• Colour in nitric acid: red
• Colour in ammonia solution: blue or purple

Examiners award 1 mark for each correct colour.

💡 Key knowledge

• Universal indicator shows pH using colours.
• Acids turn it red/orange/yellow depending on strength.
• Alkalis turn it blue/purple.
• Ammonia solution contains OH⁻ ions, so it is an alkali.

🧠 Exam technique

• Link the colour to whether the solution is an acid or alkali.
• If you do not remember the exact shade, the mark scheme usually accepts the correct colour range for the pH.

❌ Common errors

• Saying ammonia solution is green because of confusion with neutral solutions.
• Saying nitric acid is purple.
• Writing “acidic” instead of giving a colour.

✅ Correct answer

Row D

Start pH = 10, after excess nitric acid pH = 2.

💡 Key knowledge

• Ammonia solution is alkaline, so its pH is above 7.
• Adding excess acid makes the final solution acidic, so pH is below 7.
• That means the pH must fall from a high value to a low value.

🧠 Exam technique

• Look for the row that shows the pH going from alkaline to acidic.
• “Excess nitric acid” is the clue that the final pH should be low.

❌ Common errors

• Choosing a row where pH rises instead of falls.
• Forgetting that ammonia solution is alkaline.
• Thinking “excess acid” means pH still stays above 7.

📐 Calculations — step by step

1. Work out the mass of oxygen in NH₄NO₃.
   There are 3 oxygen atoms.
2. Mass of oxygen = 3 × 16 = 48
3. Relative formula mass of NH₄NO₃ = 80
4. Percentage by mass of oxygen = (48 ÷ 80) × 100
5. = 60%

✅ Correct answer

60%

Marks are for identifying the oxygen mass, using the correct total formula mass, and calculating the percentage correctly.

💡 Key knowledge

• NH₄NO₃ contains 3 oxygen atoms.
• Each oxygen atom has relative atomic mass 16.
• Percentage by mass = mass of part ÷ total mass × 100.

🧠 Exam technique

• Always show the working for calculation questions.
• Include the % sign in the final answer.
• Use the given formula mass Mᵣ = 80 if it is provided.

❌ Common errors

• Using 2 oxygen atoms instead of 3.
• Doing 16 ÷ 80 × 100 instead of using the total oxygen mass.
• Forgetting to multiply by 100.
• Writing the answer as 0.6 instead of 60% .

✅ Correct answer: what a full-mark method should include

1. Measure a fixed volume of water, for example 50 cm³, into a polystyrene cup or beaker.
2. Measure and record the starting temperature of the water.
3. Weigh a known mass of ammonium nitrate, such as 2 g, using a balance.
4. Add the salt to the water and stir until it dissolves.
5. Measure the lowest temperature reached after the salt dissolves.
6. Calculate the temperature change by subtracting the final temperature from the start temperature.
7. Repeat for different masses of ammonium nitrate and keep all other variables the same.
8. Repeat each mass several times and calculate a mean to improve reliability.

This kind of answer reaches top marks because it is clear, controlled, and directly tests how mass affects temperature change.

💡 Key knowledge

• Dissolving ammonium nitrate in water is endothermic, so the temperature should decrease.
• A fair test means changing only the mass of ammonium nitrate.
• Control variables include: volume of water, starting temperature, container, and stirring time.

🧠 Exam technique

• For 6-mark method questions, write in a logical order.
• Make it clear how you will:
  • measure
  • change one variable
  • keep others constant
  • collect temperature data
  • compare results
• Using the phrase “repeat and take a mean” often helps gain the final mark.

❌ Common errors

• Not saying how the temperature is measured.
• Not mentioning that the mass is the independent variable.
• Changing more than one thing at a time.
• Writing only “add the salt and note the temperature” — too vague for 6 marks.
• Forgetting that you must compare different masses.

📐 How to structure the result section

1. Record the initial temperature.
2. Record the final/lowest temperature.
3. Calculate temperature change = starting temperature − final temperature.
4. Compare the temperature changes for each mass.

If you include a table in your answer, label the columns clearly, e.g. mass of ammonium nitrate / g, start temperature / °C, final temperature / °C, temperature change / °C.

• It is fully methodical rather than a short list of actions.
• It includes control variables and a clear way to make the test fair.
• It shows how temperature change is obtained, not just that temperature is measured.
• It uses suitable practical language: measure, record, stir, repeat, mean.