Kinetic Particle Theory and Changes of State

Changes of state are reversible.
Substances change state because:
- When the matter is heated/cooled, the heat absorbed/given out causes the kinetic energy of the particles to change. Thus, the substance changes its state

Melting
Substance changes from solid to liquid.
Temperature which solid becomes a liquid is called melting point.
Graph is known as the heating curve
Heating curve shows how the temperature of a solid changes as it is heated to the melting point.


Freezing
Substance changes from liquid to solid
Temperature which liquid becomes a solid is called freezing point.
Graph is known as cooling curve.
Cooling curve shows how the temperature of a liquid changes as it is cooled to the freezing point.


Boiling
Substance changes from a liquid to gas
Temperature which liquid boils to become a gas is called boiling point.
- Bubbles of gas are seen when a liquid boils.

Evaporation
Liquid turns into a gas at temperatures lower than the boiling point
Particles have enough energy to escape as a gas from the surface of the liquid
Liquids that evaporate quickly at room temperature are called volatile liquids.
Example: Perfume and petrol

Similarity between Evaporation and Boiling:
Involves a liquid changing into a gas

Differences between Evaporation and Boiling:

Boiling	Evaporation
Occurs only at boiling point	Occurs at temperatures below boiling point
Occurs throughout the liquid	Occurs only at the surface of the liqiuid
Occurs rapidly	Occurs slowly

Condensation
Gas is cooled sufficiently and changes into a liquid
Water vapor touches a cold surface, condensation occurs and water droplets are obtained

Sublimation
Solid change directly into a gas.
Particles at the surface of the solid have enough energy to break away from the solid and escape as gas.
Examples: Iodine and Ammonium chloride
Substances that sublime might change directly from a gas into a solid without going through the liquid state.
- Keeps things cool