1. A solid is heated until it melts. Which statement correctly describes the change in the particles?
A
Particles gain kinetic energy, vibrate more strongly and eventually overcome the forces holding the lattice together so they can slide past each other.
B
Particles lose energy and move closer together while staying in fixed positions.
C
Particles remain completely still but the forces between them weaken.
D
Particles move freely throughout the container with very high speeds.
2. Which of the following best explains why gases are easily compressed compared to solids and liquids?
A
Gas particles are very far apart with weak forces between them, so they can be pushed closer together easily.
B
Gas particles have no mass so they take up no space.
C
Gas particles are arranged in a rigid lattice structure.
D
Gas particles move in straight lines at constant speed.
3. When steam condenses to form water, what happens to the particles and the energy?
A
Particles slow down, lose kinetic energy and move closer together as attractive forces become significant again.
B
Particles gain energy and move further apart.
C
Particles vibrate more but stay in fixed positions.
D
Particles break apart into atoms.
Particle Theory — Key Points (IGCSE)
Solids
Particles arranged in regular, repeating lattice
Strong forces of attraction between particles
Particles vibrate about fixed positions only
Low kinetic energy
Fixed shape and volume; very difficult to compress
Liquids
Particles close together but in random arrangement
Forces of attraction weaker than in solids
Particles can slide and roll past each other
Moderate kinetic energy
No fixed shape (takes container shape) but fixed volume; almost incompressible
Gases
Particles very far apart compared to solids/liquids
Extremely weak forces of attraction (often negligible)
Particles move freely and randomly in all directions at high speed
High kinetic energy
No fixed shape or volume (fills entire container); easily compressed
Energy changes: When heating, particles gain kinetic energy. At melting/boiling point, energy is used to overcome attractive forces (latent heat) rather than raising temperature.