AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2023: Question 4

📐 Step-by-Step Calculation

1. Find total atoms:
   Total = 6 (lead) + 24 (tin) = 30 atoms [1 mark]
2. Set up percentage fraction:
   (Lead atoms ÷ Total atoms) × 100 = (6 ÷ 30) × 100 [1 mark]
3. Calculate final value:
   20% [1 mark]

❌ Common Error

Dividing by the other component instead of the total:

A frequent mistake is calculating (6 ÷ 24) × 100 = 25%. You must always add both parts together to find the whole denominator.

✅ Completed Sentences

In solder the layers are distorted.

This is because the atoms of tin and lead have different sizes [1 mark].

Therefore the layers cannot easily slide (over each other) [1 mark].

💡 Key Knowledge

• Pure metals: Atoms are all identical in size. They arrange in regular layers that easily slide over one another when a force is applied (making them soft/malleable).
• Alloys: Different elements have atoms of different sizes (or diameters), which disrupts and distorts the neat regular layers.

🧠 Exam Technique: Acceptable Terms

The mark scheme accepts sizes or diameters.

For the second mark, "move over each other" is also accepted.

❌ What Not to Say

Do NOT write that atoms have different "shapes". All atoms are represented as spheres in GCSE models. Writing "shapes" scores 0 marks.

✅ Model Answer

• As percentage of tin increases, the melting point decreases [1 mark]
• To a minimum value of 183 °C (or at 62% tin) [1 mark]
• Then the melting point increases [1 mark]

🧠 Exam Technique: Graphs with Turning Points

When a graph goes down and then back up (a V-shape), your answer must have three distinct parts:

1. The initial trend (decrease)
2. The coordinate / turning point (with units!)
3. The subsequent trend (increase)

✅ Correct Answer

Melting point of pure tin = 232 °C [1 mark]

💡 How to Find Pure Tin on the Graph

• The x-axis shows: "Percentage (%) by mass of tin in the solder".
• "Pure tin" means it contains 100% tin.
• Locate 100 on the x-axis, follow up to the line, and read across to the y-axis: exactly 232 °C (6 small squares above 220, where 1 small square = 2 °C).

❌ Common Error

Reading at 0% instead of 100%. At 0% tin, you are reading the melting point of pure lead (327 °C), not tin!

✅ Correct Option

✓ The atoms gain energy and their arrangement becomes less ordered. [1 mark]

💡 Science Behind Melting

• Solid: Atoms are closely packed in a regular, orderly lattice and only vibrate about fixed positions.
• Heating / Melting: Particles absorb thermal energy, increasing their kinetic energy. Forces holding them in fixed positions are overcome.
• Liquid: Atoms can now move past each other; the arrangement becomes random and less ordered.