Explain why a gas fills the entire container it is placed in, whilst a solid keeps its own shape regardless of the container it is placed in.

OCR GCSE Chemistry A: Gateway Science (J248) — C1.1 The particle model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A scientist transfers a fixed amount of a substance, first as a solid block and then as a gas, into differently shaped containers.

Model answer (4 marks)

In a gas the particles move randomly in all directions and are far apart, so they collide with the walls and spread out to fill all the available space. In a solid the particles are arranged in a fixed, regular pattern and can only vibrate about their fixed positions; they cannot move past one another, so the solid keeps its own shape regardless of the container.

Examiner tips

Use the word ‘randomly’ and ‘far apart’ for gas; mention ‘fixed arrangement’ and ‘vibrate’ for solid.
Show the contrast: gas particles fill space, solid particles maintain shape.
Include the reason for shape retention – particles cannot move past each other.

Common mistakes

Saying the solid ‘doesn’t move’ instead of ‘vibrate about fixed positions’.
Forgetting that gas particles are far apart and collide with walls.
Using vague terms like ‘solid keeps shape’ without explaining particle arrangement.

Mark scheme (4 marks)

In a gas, particles move randomly in all directions
Gas particles are relatively spread out / far apart, so they fill all available space
In a solid, particles are held in a regular arrangement
Solid particles can only vibrate about fixed positions / cannot move past each other, so the overall shape is maintained

Key terms in this question

gas · solid