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

💡 Key knowledge

• Safe drinking water is called potable.
• Pure water has a fixed boiling point of 100°C and freezing point of 0°C.
• Dissolved solids can be found by evaporating the water to dryness and weighing what is left.
• Convert units carefully: 1000 cm³ = 1 dm³ , 1000 mg = 1 g .

🧠 Exam technique

• For 1-mark multiple choice, learn the exact scientific term.
• For practical questions, include a clear sequence: measure, heat, weigh, calculate.
• For calculations, show conversions and the final rounded answer with units.
• If asked for 2 s.f., round correctly at the end.

❌ Common errors

• Saying filtered or fresh instead of potable.
• Using a filter test for pure water — that does not test purity.
• Missing the idea of heating until dry when finding dissolved solids.
• Mixing up cm³ and dm³, or mg and g.

✅ Correct answer

Potable

💡 Key knowledge

Potable water means water that is safe for humans to drink.

❌ Common errors

Students often chose filtered because they know water may be cleaned, but the exam wants the name for water safe to drink.

✅ Correct answer

Test: Heat/boil the water.

Result: It boils at 100°C.

💡 Key knowledge

• Pure substances have sharp fixed melting/boiling points.
• For water, the boiling point is 100°C at normal atmospheric pressure.
• An alternative acceptable answer is to freeze the water and say it freezes at 0°C.

🧠 Exam technique

• The mark scheme accepted “boil water” plus “boils at 100°C”.
• Just writing “heat it” is too vague for the test mark.
• You must give a measurable result, not just “it is pure”.

❌ Common errors

• Saying “filter it” — filtering does not test purity.
• Giving the wrong temperature.
• Forgetting to include the result after the test.

✅ Correct answer

1. Weigh an empty evaporating basin/container.
2. Measure 100 cm³ of river water into it.
3. Heat the water until all the water has evaporated / until dry.
4. Weigh the container with the remaining solids.
5. Calculate the mass of dissolved solids by subtracting the empty container mass from the final mass.

💡 Key knowledge

• Dissolved solids do not disappear when water is removed.
• Evaporation leaves the dissolved substance behind.
• The mass of solids is found by difference in mass.

🧠 Exam technique

• Write the method in a logical order so the examiner can follow it.
• Use scientific wording such as evaporate or heat to dryness.
• Say weigh before and after if you want to show how to get the solid’s mass.

❌ Common errors

• Only saying “filter the water” — dissolved solids pass through a filter.
• Forgetting to measure a specific volume.
• Not explaining how the mass of solids is obtained.

📐 Calculations: step-by-step

1. Convert 250 cm³ to dm³:
   250 ÷ 1000 = 0.25 dm³
2. Convert 125 mg to g:
   125 ÷ 1000 = 0.125 g per dm³
3. Multiply concentration by volume:
   0.25 × 0.125 = 0.03125 g
4. Round to 2 significant figures:
   0.031 g

✅ Correct answer

0.031 g

💡 Key knowledge

• cm³ → dm³: divide by 1000.
• mg → g: divide by 1000.
• Concentration in mg per dm³ means mass for 1 dm³, so scale by the volume in dm³.

❌ Common errors

• Using 250 directly instead of converting to dm³.
• Forgetting to convert mg to g.
• Giving 0.03125 g and not rounding to 2 s.f.
• Mixing up multiplication and division.

📐 Calculations: step-by-step

1. Use the formula:
   percentage = (actual ÷ maximum) × 100
2. Substitute the values:
   (44 ÷ 500) × 100
3. Calculate:
   = 8.8%

✅ Correct answer

8.8%
or 9% for rounded answers accepted by the mark scheme.

💡 Key knowledge

• “Of the maximum allowed” means compare the sample with the limit.
• Always use the same units: both values are already in mg per dm³.

❌ Common errors

• Doing 500 ÷ 44 instead of 44 ÷ 500 .
• Forgetting to multiply by 100.
• Writing 0.088 instead of 8.8%.

✅ Facts to learn

• Safe drinking water = potable.
• Pure water boils at 100°C.
• Pure water freezes at 0°C.
• Dissolved solids can be found by evaporation to dryness.

🧠 Skills to practise

• Writing a clear practical method.
• Converting units correctly.
• Using ratio/percentage calculations carefully.