Home » Handbook of O Level Physics » UNIT 11: MELTING AND BOILING

# UNIT 11: MELTING AND BOILING

• Melting point: It is the fixed temperature at which a solid changes into a liquid.

• Freezing point: It is the fixed temperature at which a liquid changes into a solid.

Note: Melting point and freezing point are the same temperature. For water it is 0oC.

• Boiling point: It is the fixed temperature at which a liquid changes into a gas.

• Condensation point: It is the fixed temperature at which a gas changes into a liquid.

Note: Boiling point and condensation point are the same temperature. For water it is 100oC.

• Changes of state

• Melting: It is the change of state from solid to liquid without a change in temperature.

• Solidification / Freezing: It is the change of state from liquid to solid without a change in temperature.

• Boiling: It is the change of state from liquid to gas without a change in temperature.

• Condensation: It is the change of state from gas to liquid without a change in temperature.

• Evaporation: It is the change of state from liquid to gas at any temperature.

• Factors that affect melting and boiling points are:

 Action Freezing / melting point Boiling point Add impurity Decreases Increases Add pressure Decreases Increases
• Latent heat means hidden heat. It is because as long as a change of state is going on, the heat supplied or taken does not produce a change in temperature.

• Latent heat of fusion: It is the heat energy required to change a solid to liquid or vice versa without any change in temperature.

• Unit: J

• Specific latent heat of fusion: It is the heat energy required to change 1 kg of a solid to liquid or vice versa without a change in temperature.

l = E / m

• Unit: J/kg

• Latent heat of vaporisation: It is the heat energy required to change a liquid to vapour or vice versa without any change in temperature.

• Unit: J

• Specific latent heat of vaporisation: It is the heat energy required to change 1 kg of a liquid to gas or vice versa without any change in temperature.

l = E / m

• Unit: J/kg

• Comparison between boiling and evaporation

• Unlike boiling, evaporation happens at any temperature.

• Unlike boiling, evaporation only happens at the surface.

• Unlike boiling, evaporation is a slow process.

• Unlike boiling, evaporation does not cause bubbling.

• Unlike boiling, evaporation may not require a special heat source. The heat required for evaporation can come from the surroundings.

• Evaporation causes a decrease in temperature because higher energy molecules escape from the surface and the average kinetic energy of the molecules left in the liquid comes down.

• A refrigerator uses evaporation and condensation to work.

• Freon condenses (when compressed) to release heat and evaporates when (decompressed) to absorb heat.

• Factors that increase the rate of evaporation are:

• higher temperature

• less humidity (which refers to the amount of moisture in the air)

• more surface area

• movement of air

• lower atmospheric pressure

• using another liquid with a lower boiling point