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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


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