AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2018: Question 3

💡 Key knowledge

• Pure water contains only H₂O, with no dissolved substances.
• Potable water is safe to drink, but it can still contain dissolved salts and chlorine.
• Seawater must have salt removed before it can be drunk.
• Groundwater usually needs filtering and sterilising.

🧠 Exam technique

• Use the data in the table carefully.
• For short 1-mark answers, be precise and direct.
• For calculations, show the method if possible and include units.

❌ Common errors

• Saying potable water is “pure” — it is not.
• Calling seawater treatment “filtration” only — dissolved salt is still there.
• Forgetting to scale from 5 cm³ to 1000 cm³ correctly.

✅ Correct answer

X = 16.0 g (accept 16)

📐 Calculation steps

1. In 5 cm³ of river water, dissolved solids = 0.08 g .
2. 1000 cm³ is 1000 ÷ 5 = 200 times larger.
3. So multiply the mass by 200: 0.08 × 200 = 16.0 .
4. Answer with unit: 16.0 g .

🧠 Exam technique

• Use proportional reasoning: if volume increases by 200, mass of dissolved solids increases by 200.
• Keep units consistent: cm³ and g.
• One mark means the final answer must be correct.

❌ Common errors

• Adding instead of multiplying.
• Using 1000 ÷ 5 incorrectly.
• Writing 1.6 g or 0.16 g from a decimal slip.

✅ Correct answers

• Advantage: more accurate result
• Disadvantage: takes longer / more energy needed to heat more water

💡 Key knowledge

• A larger sample can reduce percentage uncertainty.
• More water means more time to evaporate off the water.
• Heating a bigger volume uses more energy.

🧠 Exam technique

• For “give one advantage and one disadvantage”, you need two separate points.
• Keep each point short and practical.
• If you write “more accurate”, that is enough for the mark.

❌ Common errors

• Saying “more reliable” — this is not accepted here.
• Writing “expensive” without linking to heating energy.
• Giving two advantages or two disadvantages instead of one of each.

✅ Correct answer

Pure water has no dissolved solids / impurities or no dissolved chlorine.
Potable water has dissolved solids / impurities or has dissolved chlorine.

💡 Key knowledge

• Pure water = only H₂O.
• Potable water = safe to drink, but not necessarily pure.
• Potable water may contain added chlorine to kill microorganisms.

🧠 Exam technique

• Use a contrast word: “pure water has…, potable water has…”.
• Short definition-style answers work best for 1 mark.
• Do not drift into “safe to drink” unless you also mention dissolved substances.

❌ Common errors

• Saying potable water is pure.
• Only saying “potable water is safe to drink” without comparing it to pure water.
• Not mentioning dissolved solids, impurities, or chlorine.

✅ Correct answers

Groundwater:

• filtered
• sterilised

Seawater:

• distilled or reverse osmosis

💡 Key knowledge

• Filtering removes insoluble solids.
• Sterilising kills microorganisms.
• Reverse osmosis removes dissolved salts using a membrane and pressure.
• Distillation can also separate water from dissolved salts.

🧠 Exam technique

• This is a 3-mark answer, so you need three clear treatment points.
• Make sure groundwater and seawater are treated differently.
• Use the correct science words: filtered, sterilised, distilled, reverse osmosis.

❌ Common errors

• Saying seawater is “filtered” only.
• Saying “boiled” without explaining that water must be separated from salt.
• Writing “fractional distillation” — this is not accepted here.

✅ Correct answer

Mass of dissolved solids = 0.143 g

Accept 0.14 g for full marks.

📐 Calculation steps

1. Write the percentage as a decimal fraction: 2.2 ÷ 100 = 0.022
2. Multiply by the total mass: 0.022 × 6.50
3. Calculate: 0.143
4. State the unit: 0.143 g

🧠 Exam technique

• Percentage means “out of 100”, so divide by 100 first or use the decimal form.
• Keep the answer in grams because the question asks for mass.
• Use the given precision carefully; 0.14 g is acceptable because of rounding.

❌ Common errors

• Multiplying by 2.2 instead of 0.022.
• Giving 0.022 g, which is the percentage as a decimal but not the mass.
• Forgetting the unit or writing the wrong unit.
• Rounding too early and losing accuracy.

✅ Top-level features

• Used exact scientific terms such as filtered, sterilised, reverse osmosis, and dissolved solids.
• Answered the question directly without extra waffle.
• Showed the scaling idea clearly in calculations.

💡 Quick memory aid

• Pure = only water
• Potable = safe to drink
• Groundwater = filter + sterilise
• Seawater = desalinate

❌ Most common traps

• Using the wrong treatment for seawater.
• Confusing “safe to drink” with “pure”.
• Not scaling the 5 cm³ result up to 1000 cm³ properly.