Question 1

The student determined the concentration of sodium chloride in the salt solution using this method:
1. Weigh an empty evaporating dish.
2. Add 25.0 cm³ of the salt solution into the evaporating dish.
3. Heat the evaporating dish and contents.
4. Weigh the evaporating dish and contents.
5. Repeat steps 3 to 4 until there is no further change in mass.
6. Repeat steps 1 to 5 three more times.

Why did the student heat the evaporating dish and contents until the mass did not change?

Accepted Answer

Consider the purpose of heating the solution. The goal is to leave only the solid salt behind. How can the student be sure that all the liquid has gone? The process is called heating to a constant mass.

Question 2

What mistakes do candidates make on this AQA GCSE question?

Accepted Answer

✗ Saying "to make sure the reaction is complete" (it's a physical separation, not a reaction).
✗ Saying "to remove impurities".
✗ Giving a vague answer like "to get an accurate result" without explaining why.

How to approach this question

Full Answer

Common mistakes

Practice the full AQA GCSE Chemistry Higher Tier Paper 2

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