IGCSE Physics - 2.2.3 Melting, Boiling and Evaporation

Condensation and Solidification: Particle Level Explanation

This section explores the processes of condensation and solidification, focusing on how the arrangement and movement of particles change during these phase transitions. Understanding these processes requires considering the kinetic theory of matter.

Condensation

Condensation is the process where a gas changes into a liquid. This occurs when the kinetic energy of the gas particles decreases.

Particle Explanation:

1. In a gas, particles move randomly and have high kinetic energy.
2. When the kinetic energy of gas particles is reduced (e.g., by decreasing the temperature), they move more slowly.
3. As the particles slow down, the attractive forces between them become more significant.
4. These attractive forces cause the gas particles to cluster together.
5. As more particles cluster, they form a liquid. The particles are still close together but are now held by intermolecular forces.

Key Points:

• Condensation occurs at the boiling point of a gas.
• The temperature at which condensation occurs is called the condensation point.
• Examples: Water droplets forming on a cold glass, dew forming on grass.

Solidification

Solidification is the process where a liquid changes into a solid. This occurs when the attractive forces between the liquid particles become strong enough to hold them in fixed positions.

Particle Explanation:

1. In a liquid, particles have sufficient kinetic energy to move past each other, but they are still attracted to each other.
2. When the temperature of a liquid is decreased, the kinetic energy of the particles decreases.
3. As the particles lose kinetic energy, the attractive forces between them become more dominant.
4. The particles are then able to arrange themselves into a more ordered, fixed arrangement, forming a solid.
5. In a solid, particles vibrate about fixed positions but do not move freely.

Key Points:

• Solidification occurs at the freezing point of a liquid.
• The temperature at which solidification occurs is called the freezing point.
• Examples: Water freezing into ice, metals solidifying when cooled.

Phase Change	Particle Arrangement	Kinetic Energy	Intermolecular Forces
Condensation	Particles cluster together, but still have some freedom of movement	Decreases	Increase
Solidification	Particles arrange into a fixed, ordered structure	Decreases	Increase

Suggested diagram: A diagram showing gas particles with high kinetic energy and random movement transitioning to liquid particles with lower kinetic energy and more clustered movement, then to solid particles with very low kinetic energy and a fixed arrangement.