AQA GCSE Physics Physics Paper 1 (Higher), June 2025: Question 5

✅ Model Answer

Atoms of an element with:

• The same number of protons [1 mark]
• A different number of neutrons [1 mark]

💡 Key Knowledge

• Proton number (Z): Defines the identity of the chemical element.
• Neutrons (N): Stabilise the nucleus; varying their number changes mass but not chemical properties.
• Electrons are not part of the core definition of isotopes—focus strictly on the nucleus!

🧠 Exam Technique

Always state both parts clearly: same protons AND different neutrons. Avoid simply saying "different mass" without specifying that it is due to different numbers of neutrons.

❌ Common Errors

• Confusing neutrons and electrons (mentioning varying electrons will lose you credit).
• Vaguely stating "atoms with different weights" rather than using the term neutrons or mass number.

✅ Model Answer

• Two light nuclei fuse / join together [1 mark]
• To form a single heavier nucleus (e.g. hydrogen fusing into helium) [1 mark]
• Some of the mass is converted into energy [1 mark]

💡 Key Knowledge

• The total mass of the resulting nucleus is slightly less than the combined mass of the original individual nuclei.
• This missing mass (mass defect) is radiated away as energy, governed by Einstein's mass-energy equation: E = mc².

❌ Common Errors

• Confusing fusion (joining light nuclei) with fission (splitting heavy nuclei like Uranium-235).
• Saying "atoms fuse" instead of nuclei fuse. Fusion occurs at extreme temperatures where electrons are stripped away (plasma).
• Writing that "energy is created" (which violates the law of conservation of energy) instead of stating mass is converted into energy.

🧠 Exam Technique

Structure your 3-mark answer chronologically:

1. Start: Light nuclei (e.g., hydrogen isotopes) join.
2. End product: A heavier nucleus (e.g., helium).
3. The reason for energy release: A loss in nuclear mass converted to energy.

✅ Model Answer

• Absorbing a large amount of thermal energy causes only a small temperature increase in the walls [1 mark]
• Therefore, the walls of the reactor do not melt (or reach their melting point) [1 mark]

📐 Concept Link: ΔE = m c Δθ

Rearranging the specific heat capacity formula shows how Δθ relates to c:

Δθ = ΔE / (m × c)

When c (specific heat capacity) is very large, the resulting temperature rise (Δθ) is very small for a given input of energy (ΔE).

🧠 Exam Technique

This is a 2-step cause-and-effect question:

• Cause: High c → smaller temperature increase for high energy input.
• Effect: Prevents structural failure / melting of reactor walls.

❌ Common Errors

• Confusing specific heat capacity with thermal conductivity (saying it "stops heat escaping").
• Stating that a high specific heat capacity means the material "can absorb lots of heat" without linking it to a small rise in temperature.

✅ Model Answer

Any one from:

• A nuclear explosion [1 mark]
• A nuclear meltdown [1 mark]
• A nuclear bomb [1 mark]

❌ Common Errors

• Giving vague answers like "it gets hot", "it breaks", or "fire".
• Writing "radiation leak" without referring to the event causing it (a meltdown or explosion).