AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), June 2025: Question 4

✅ Correct Answer

Most metals are found in the Earth as compounds.

💡 Key Knowledge

Only very unreactive metals (like gold and platinum) are found in the Earth's crust as native elements. Most metals react readily with oxygen and other non-metals over geological time to form metal compounds (such as oxides and sulfides) found in rocks known as ores.

✅ Correct Answer

☑ Iron is less reactive than carbon.

💡 Key Knowledge

A more reactive element displaces a less reactive element from its oxide:

2Fe₂O₃ + 3C → 4Fe + 3CO₂

Because carbon is above iron in the reactivity series, carbon takes the oxygen away (reduces iron oxide).

❌ Common Errors

Ticking "Iron is more reactive than carbon". If iron were more reactive, carbon could not remove the oxygen, and electrolysis would be required instead (as with aluminium).

✅ Correct Answer

☑ To reduce the amount of energy used

💡 Key Knowledge

Pure aluminium oxide (Al₂O₃) has an extremely high melting point (over 2000 °C). Dissolving it in molten cryolite drops the operating temperature to around 950 °C. This saves immense amounts of thermal energy and dramatically lowers operating costs.

🧠 Exam Technique

Watch out for distractor options like "reduce the rate of reaction" or "reduce purity". Lowering the melting point specifically conserves fuel/electricity, keeping energy consumption and costs down.

✅ Correct Answer

☑ The electrodes react with oxygen gas.

💡 Key Knowledge

Negative oxide ions (O²⁻) are attracted to the positive electrode (anode) where they lose electrons to form oxygen gas (O₂):

2O²⁻ → O₂ + 4e⁻

At high operating temperatures (~950 °C), the graphite (carbon) electrodes react with this oxygen to produce carbon dioxide gas:

C (s) + O₂ (g) → CO₂ (g)

This causes the anodes to burn away slowly, requiring regular replacement.

❌ Common Errors

Selecting "The electrodes melt". Graphite has a giant covalent lattice with a melting point above 3600 °C, so it does not melt at 950 °C; it burns away chemically.

✅ Correct Answer

☑ The alloy is harder than the pure metal.

💡 Key Knowledge

• Pure metal: Identical atoms are arranged in regular layers that slide easily over one another when a force is applied.
• Alloy: Contains atoms of different sizes. This disrupts the regular layers, making it harder for layers to slide.

✅ Model Answer (Full 3 Marks)

As the percentage of silver increases, the melting point of the alloy decreases [1 mark] until reaching a minimum at 70% silver (or 530 °C) [1 mark], and then increases [1 mark].

📐 Step-by-Step Data Reading

1. Identify Phase 1 (Initial Trend): From 0% silver (660 °C), the curve slopes downwards → melting point decreases.
2. Pinpoint the Turning Point: The bottom of the dip is exactly at 70% on the horizontal axis and between 520 °C and 540 °C (read as 530 °C) on the vertical axis.
3. Identify Phase 2 (Final Trend): Past 70% silver, the line slopes sharply upwards to 100% silver (960 °C) → melting point increases.

🧠 Exam Technique: Two-Part Trends

Whenever a curve goes down and then goes up (a "U-shape"), an answer stating only "it decreases and increases" will lose marks unless you state the exact turning point coordinate.

• Always state both the direction before and the direction after.
• Always quote the turning point value with appropriate units (% or °C).

❌ Common Traps to Avoid

• Writing "it increases overall" without mentioning the initial decrease.
• Misreading the turning point percentage (e.g. guessing 75% instead of 70%). The mark scheme strictly allows 69% to 71% or 520 °C to 540 °C.
• Forgetting to include numbers when the command states "Use data from Figure 9".