AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2024: Question 6

✅ Correct answer

Large amounts of energy are required.

💡 Key knowledge

• Distillation involves heating seawater so that water evaporates, then condenses again.
• Heating and boiling use a lot of energy.
• The key idea is that distillation is energy-intensive.

🧠 Exam technique

• Keep it short: one clear disadvantage is enough for 1 mark.
• Use science language such as energy , heating , or expensive to run .

❌ Common errors

• Writing “it takes a long time” or “it is expensive” alone — the mark scheme says these are ignored.
• Talking about desalination without stating a disadvantage.

✅ Correct answers

Reverse osmosis

Using membranes

💡 Key knowledge

• Reverse osmosis pushes seawater through a membrane.
• The membrane lets water molecules pass through but not dissolved salts.
• This is a common way to desalinate seawater.

🧠 Exam technique

• If the question says “describe”, give a little extra detail: name the method and explain how it works.
• A full 2-mark answer could be:
  Reverse osmosis uses a membrane to remove dissolved salts from seawater.

❌ Common errors

• Repeating distillation instead of giving a different method.
• Saying only “desalination” — too vague on its own for the method mark here.
• Not mentioning membranes when describing reverse osmosis.

✅ Correct answer

Heat it until the mass is constant so that all the water has evaporated.

💡 Key knowledge

• If any water is left, the measured mass of dissolved solids will be too high or unreliable.
• Constant mass means the result stops changing after repeated heating and weighing.
• This is a standard way to make sure evaporation is complete.

🧠 Exam technique

• Use the phrase constant mass — it is the key mark point.
• For the second mark, explain the reason: all water has gone.
• A top answer might say:
  Repeat heating and weighing until the mass stays the same, so all the water has evaporated.

❌ Common errors

• Saying only “repeat for accuracy” — too vague.
• Forgetting to mention that the aim is to remove all the water.
• Thinking the evaporating basin should be weighed once only.

📐 Calculations — step by step

1. Identify the percentage of sodium chloride: 77.8%
2. Find 77.8% of the total dissolved solids mass:
   (77.8 ÷ 100) × 3.50
3. Work it out:
   0.778 × 3.50 = 2.723
4. Round appropriately:
   2.72 g (also accepts 2.723 g)

✅ Correct answer

Mass of sodium chloride = 2.72 g

💡 Key knowledge

• Percentage means “out of 100”.
• Always multiply the total mass by the decimal form of the percentage.
• Keep an eye on units: the answer is a mass, so use g.

❌ Common errors

• Using 77.8 × 3.50 without dividing by 100.
• Giving the answer in cm³ instead of g.
• Rounding too early and losing accuracy.

✅ Correct answers

Displacement using (scrap) iron

Electrolysis

💡 Key knowledge

• Leachate is a solution containing dissolved copper compounds.
• Copper can be removed by a more reactive metal such as iron, or by electrolysis.

🧠 Exam technique

• Give two distinct methods.
• “Scrap iron” is acceptable because the mark scheme allows it.
• If you write “displacement”, that is enough if the method is clear.

❌ Common errors

• Writing “filtration” or “evaporation” — these do not remove dissolved copper ions here.
• Only stating “reduction” without explaining the method.

✅ Correct answer

Plants are harvested and burned to produce ash, which contains the metal compounds.

💡 Key knowledge

• Phytomining uses plants to absorb metal compounds from low-grade ore.
• After growth, the plants are collected.
• Burning removes most of the plant material and leaves ash that contains the useful compounds.

🧠 Exam technique

• This is a process question, so answer in the correct sequence.
• Use the exact idea of ash containing metal compounds for the final mark.
• A full answer could be:
  The plants are harvested, then burned. The ash left behind contains the metal compounds.

❌ Common errors

• Stopping at “the plants are harvested” — not enough for full marks.
• Saying the metal is extracted directly from the living plant without burning.
• Forgetting that the useful material is in the ash.

📐 Calculations — step by step

1. Use the rate: 215 kg nickel per 1 hectare
2. Find the number of hectares needed:
   area = 750 ÷ 215
   area = 3.49 hectares
3. Convert hectares to m²:
   3.49 × 10 000 = 34 900 m²

Answer: 34 900 m²

✅ Correct answer

Area required = 34 900 m²

💡 Key knowledge

• Use proportion: more nickel needs a larger area.
• 1 hectare = 10 000 m²
• Keep units consistent and convert at the end.

❌ Common errors

• Dividing by 10 000 instead of multiplying.
• Forgetting to convert hectares into m².
• Rounding to a badly chosen value too early.
• Mixing up the ratio: it is 215 kg per hectare, not the other way round.

🧠 Exam technique

• Show the proportional reasoning clearly.
• Write units in every step: kg, hectares, m².
• A top-level response would show both the calculation and the conversion.

💡 Core facts

• Distillation uses lots of energy.
• Reverse osmosis uses membranes.
• Constant mass means all the water has evaporated.
• Percentage mass = percentage ÷ 100 × total mass.
• Copper can be extracted by displacement with iron or electrolysis.
• Phytomining: harvest plants, burn them, collect the ash.
• 1 hectare = 10 000 m².

🧠 How to gain full marks

• Use the exact scientific phrase the mark scheme wants where possible.
• For calculations, show the method and include units.
• For 2- and 3-mark questions, give complete process steps in order.