AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2023: Question 4

✅ Mark Scheme Answers

Any two of the following points (1 mark each):

• Water (vapour) is colourless (allow: cannot be seen).
• Both calcium hydroxide and calcium oxide are white powders / solids (or: there is no change in the appearance of the solid).

🧠 Exam Technique: Use the Table!

The prompt explicitly commanded: "Use the information in Table 3."

Never write generic answers like "atoms are rearranged". Look directly at the table columns: reactant solid is white, product solid is white, and the gas produced is invisible/colourless.

❌ Common Errors

• Writing "a gas was made" without mentioning that it is colourless / invisible.
• Stating that "nothing was produced" (products are formed, they just look identical or escape invisibly).

💡 Key Knowledge

A chemical reaction is usually identified by observations like colour changes, effervescence (bubbles in liquids), or temperature changes. When a white solid breaks down into another white solid and a colourless gas, no visible change occurs in the tube.

✅ Mark Scheme Answer

Any one of the following (1 mark):

• The stopper would be pushed out / pop off.
• The test tube may break / shatter / burst.
• Allow: pressure would build up inside the tube.

❌ Common Errors

• Vague answers such as "it is dangerous" without specifying why.
• Claiming the rubber stopper would just "melt" (while possible, this does not address the main physical hazard of gas pressure).

🧠 Exam Technique: Subtraction Logic

To find the mass of the contents inside a container:

Mass of contents = (Mass of tube + contents) − (Mass of empty tube)

Without knowing what the empty glass test tube weighs, you cannot find the exact mass of powder inside it.

❌ Incorrect Options

• "The change in mass at the end" – this is a calculated result, not a direct measurement needed after each minute.
• "The mass of contents at start" – this was already given in the question as 2.00 g.

💡 What is "Heating to Constant Mass"?

Look at Table 4:

• At 4 minutes: 1.52 g
• At 5 minutes: 1.51 g
• At 6 minutes: 1.51 g

The mass stops decreasing at 5 minutes. This proves all water vapour has escaped and the reaction has finished.

❌ Common Error: Choosing 6 minutes

Students often choose 6 minutes because it is the end of the table. However, the question asked for the minimum time. The mass had already reached its final constant value at 5 minutes.

✅ Question 04.6 Answer

Tick box: ☑ 5.90 kJ (1 mark)

✅ Question 04.7 Answer

The forward reaction is endothermic (1 mark)

💡 Key Principle: Reversible Reactions & Energy

For any reversible reaction:

• If the forward reaction is endothermic (takes in energy, +5.90 kJ), the reverse reaction must be exothermic (releases energy, −5.90 kJ).
• Energy cannot be created or destroyed (Law of Conservation of Energy), so the numerical value (5.90 kJ) stays identical!

❌ Common Errors

• Selecting "Less than 5.90 kJ" thinking that energy is "lost" or reaction wasn't 100% efficient.
• Confusing endothermic and exothermic. Remember: Exothermic reactions exit heat (transfer energy to surroundings).