AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 3

✅ Correct Answer

• Data comparison (1 mark):
  Total mass before = 78.26 + 78.50 = 156.76 g
  Total mass after = 108.22 + 48.54 = 156.76 g
  (Alternatively: Beaker A gained 29.96 g and Beaker B lost 29.96 g)
• Conclusion (1 mark):
  The mass of products equals the mass of reactants (or there is no overall change in mass / no atoms were lost or made).

🧠 Exam Technique

The prompt explicitly says: "You should use data from Table 3 in your answer."

Whenever you see this instruction, you must calculate values. Stating only "mass is neither created nor destroyed" earns zero marks without calculating and comparing total masses before and after.

❌ Common Errors

• Only calculating the mass before (156.76 g) but failing to calculate and write down the total mass after.
• Looking only at Beaker A and concluding "mass increased from 78.26 g to 108.22 g, so mass was gained". You must consider both beakers together!

💡 Key Knowledge

In a closed or precipitation system where no gas escapes, total mass of reactants = total mass of products because atoms are simply rearranged, not created or destroyed.

✅ Correct Answer

Filtration (or filter / filtering through filter paper and a funnel).

💡 Key Knowledge

Look at the state symbols in the equation:
AgNO₃(aq) + NaI(aq) → AgI(s) + NaNO₃(aq)

Because silver iodide is a solid (s) and sodium nitrate is dissolved in solution (aq), filtration is the standard method to separate an insoluble solid from a liquid mixture.

✅ 03.3: Impurity Removed by Rinsing

Any one of:

• Sodium nitrate (or NaNO₃ solution)
• Silver nitrate (excess AgNO₃ reactant)
• Sodium iodide (excess NaI reactant)
• Allow individual ions: sodium / nitrate / silver / iodide ions

✅ 03.4: Reason for Warming

• To remove / evaporate the water
• To dry the solid precipitate

🧠 Exam Technique: Practical Insight

When an insoluble precipitate is filtered, the filter paper retains wet solid (residue). The liquid clinging to it contains dissolved soluble salts. Rinsing with distilled water washes these dissolved salts away without dissolving the precipitate. Gentle warming then removes the water to obtain a dry sample.

❌ Common Errors

• 03.3: Writing "silver iodide" (that is the desired product, not an impurity!).
• 03.4: Stating "to speed up the reaction" — the reaction is already finished!

❌ Common Calculation Traps

• Significant figures penalty: Leaving the answer as 73% (2 s.f.) or 73.44% (4 s.f.) loses the 4th mark. Always check the required precision!
• Wrong denominator: Dividing 235 by only one reactant (e.g. 235 / 170) instead of the sum of all reactants.

🧠 Error-Carried-Forward (ECF)

If you made an addition error in Step 1 (e.g. finding a total M_r of 310), you could still earn Marks 2, 3, and 4 if you correctly applied your number and rounded your final answer to 3 significant figures.

✅ Acceptable Answers (Give ONE)

• For sustainable development (conserves raw materials / resources).
• For economic reasons (reduces cost of purchasing raw materials / higher profit).
• To reduce waste (less unwanted by-product to treat or dispose of).
• To produce a higher percentage of useful product.

❌ Common Errors & Misconceptions

• Confusing atom economy with yield: Saying "it gives a higher yield" is incorrect. Yield refers to practical recovery; atom economy is theoretical waste from the chemical equation.
• Vague answers: Simply writing "it is cheaper" without clarifying why (e.g. less wasted materials, less waste disposal costs) might not gain full credit.