AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2024: Question 5

✅ Correct answers

• Irradiating: radiation enters the water, so the water is exposed to radiation.
• This does not make the water radioactive, so it does not increase the hazard in the same way as contamination.
• Contaminating: radioactive isotopes enter the water / the body.
• This increases the hazard because the source is now inside the water you drink.

💡 Key knowledge

• Irradiation means exposure to radiation from a source.
• Contamination means radioactive material gets onto or into something.
• In this question, the revigator walls contain the radioactive source, so the water can be irradiated without becoming radioactive.
• If radioactive atoms get into the water, then the water is contaminated and the hazard is greater.

🧠 Exam technique

• Use the command word: Explain means give a reason, not just a definition.
• To score both marks, link the idea to hazard.
• Good wording: “radiation enters the water, but the water does not become radioactive”.
• For contamination, mention that radioactive isotopes enter the water or body.

❌ Common errors

• Writing that irradiation and contamination mean the same thing.
• Saying irradiation “makes the water radioactive” — this is wrong.
• Not mentioning the effect on hazard.
• Using vague phrases like “it gets dangerous” without explaining why.

✅ Correct answer

²³₅ V → ²⁴₂ Cr + ⁰₋₁ β

This is the correct option because beta decay increases the atomic number by 1, but the mass number stays the same.

💡 Key knowledge

• A beta particle is written as ⁰₋₁ β .
• In beta decay, a neutron changes into a proton.
• That means:
  • mass number stays the same,
  • atomic number increases by 1.
• So vanadium-52 becomes chromium-52.

🧠 Exam technique

• Check both sides of the equation balance.
• Mass numbers must add up: 52 = 52 + 0.
• Atomic numbers must add up: 23 = 24 + (−1).
• If you are unsure, remember: beta minus = atomic number +1.

❌ Common errors

• Changing the mass number to 51 or 53.
• Using the wrong daughter nucleus, such as titanium or potassium.
• Forgetting the beta particle is on the product side.
• Mixing up alpha and beta decay rules.

📐 Calculations / reasoning

Step 1: Compare the time passed with each half-life.

• Radon-222 half-life = 3.8 days
• Vanadium-52 half-life = 3.7 minutes
• Time elapsed = 7.4 minutes

Step 2: Decide how many half-lives have passed.

• For radon-222, 7.4 minutes is a very small fraction of 3.8 days, so activity is almost unchanged.
• For vanadium-52, 7.4 minutes is exactly 2 half-lives because 7.4 ÷ 3.7 = 2.

Step 3: State the effect on activity.

• Radon-222 activity remains almost the same.
• Vanadium-52 activity decreases by a factor of 4.

✅ Correct answers

• Radon-222: activity remained (almost) the same because 7.4 minutes is much smaller than 3.8 days.
• Vanadium-52: activity decreased by a factor of 4 because 2 half-lives had passed.

💡 Key knowledge

• After 1 half-life, activity halves.
• After 2 half-lives, activity becomes one quarter.
• When the time is tiny compared with the half-life, the activity changes very little.

🧠 Exam technique

• Always compare the time with the half-life before deciding the change.
• Use “factor of 4” rather than “decreases a lot” for full credit.
• Make the link: time passed → number of half-lives → change in activity.

❌ Common errors

• Saying radon-222 changes a lot just because any time has passed.
• Missing that 7.4 minutes is tiny compared with 3.8 days.
• For vanadium-52, saying the activity halves instead of quartering.
• Forgetting that 2 half-lives means the activity is divided by 2 twice.

✅ Correct answer

Peer review

💡 Key knowledge

• Peer review means other scientists check the methods, data, and conclusions before publication.
• It helps spot mistakes and improve reliability.

🧠 Exam technique

• For 1 mark, the exact term is enough: peer review.
• Do not write vague answers like “testing” or “checking” on their own.

• Irradiation = exposed to radiation; contamination = radioactive material gets into the water/body.
• Beta decay keeps the mass number the same and increases atomic number by 1.
• If time is much less than the half-life, activity changes very little.
• Peer review = other scientists check the work before it is published.