How to Answer 'Explain' Questions in IGCSE Physics

Use this guide to build explanations that follow a logical chain and earn marks in Cambridge IGCSE Physics 0625 exams.

What the examiner expects

‘Explain’ means give reasons. The mark scheme awards marks for a chain of cause and effect, not for restating the observation.

The cause-effect-consequence structure

For any ‘Explain’ question, build a chain:

1. Cause: What starts the process? (e.g. “temperature increases”)
2. Effect: What physics happens as a result? (e.g. “particles move faster, so collisions with walls are harder and more frequent”)
3. Consequence: What is the observable outcome? (e.g. “pressure increases”)

Each link in the chain is typically worth one mark.

Worked example: “Explain why a gas exerts more pressure when heated at constant volume.” [3 marks]

1. Heating increases the average kinetic energy of the gas particles. [cause]
2. Particles move faster, hitting the container walls harder and more frequently. [effect]
3. The total force on the walls increases, so the pressure increases (since volume and area are constant). [consequence]

Worked example: “Explain why metals are good conductors of thermal energy.” [2 marks]

1. Metals contain free (delocalised) electrons.
2. These electrons can move through the lattice and transfer kinetic energy rapidly from the hot end to the cold end.

Common errors and how to correct them

1. Restating the question. “It conducts because it is a good conductor” earns zero. You must explain WHY.

2. Missing the physics link. Going straight from cause to consequence without the mechanism. “Temperature increases so pressure increases” skips the particle explanation.

3. Vague language. “The molecules move around more” is less precise than “particles have greater average kinetic energy and collide with the walls harder and more frequently.”

How to apply this in an exam

Count the marks. Write one clear point per mark. Use precise physics language (name the quantity, the mechanism, the principle). Check that each point logically follows from the previous one.