Specific Latent Heat

• Whilst the substance is changing state (e.g. from liquid to gas), the temperature of the substance remains constant, despite the fact that energy is still being transferred to the substance
• Molecules in a solid


  • Tightly bound together
• Molecules in a liquid


  • Free to flow over one another
• Molecules in a liquid


  • Less tightly bound together
• Molecules in a gas

  • Free to move completely apart from one another
• To change the state from liquid to gas


  Molecules need to gain enough energy to overcome the intermolecular forces of attraction holding them close together in their liquid structure
• When a substance changes state, its temperature remains constant, even though energy is being transferred to or away from the substance
• Specific Latent Heat


  The amount of energy required to change the state of 1 kg of a substance with no change in temperature
• Specific latent heat of fusion


  The energy required to convert 1 kg of a substance between a solid and a liquid state with no change in temperature
• Melting a solid


  1. Temperature stays constant until all of the substance has melted

  2. Latent heat of fusion is the energy needed per kg for all the particles to overcome intermolecular forces of attraction in the solid state
• Freezing a liquid


  1. Substance will solidify at the same temperature as its melting point

  2. Latent heat of fusion is the energy per kg transferred away until all particles succumb to intermolecular forces of attraction in the solid state
• Specific latent heat of vaporisation

  The energy required to convert 1 kg between a liquid and a gaseous state with no change in temperature
• Vaporising a liquid


  1. Temperature will stay constant until all of the substance has vaporised

  2. Latent heat of vaporisation is the energy per kg needed for all particles to overcome intermolecular forces of attraction in the liquid state
• Condensing a gas


  1. Substance will condense at the same temperature as its boiling point

  2. Latent heat of vaporisation is the energy per kg transferred away until all particles succumb to intermolecular forces of attraction in the liquid state