Heat of Fusion and Heat of Vaporization

How much energy is released during phase changes?

The heat of fusion of a substance tells us how much thermal energy is absorbed when 1 mol of that substance changes from a solid state to a liquid state. The heat of solidification is the reverse phase change. It tells us how much thermal energy is released when 1 mol of a substance changes from a liquid to a solid. Each phase change has its own term for how much energy is released or absorbed when 1 mol of a substance undergoes that phase change. It is important to note that each of these terms only applies as long as the temperature of the substance is not changing.


Term Phase Change
Energy Released or Absorbed
Amount of Energy (kJ/mol)
heat of fusion
solid to liquid
absorbed 6.01
heat of solidification
liquid to a solid
released 6.01
heat of vaporization
liquid to a gas
absorbed 40.66
heat of condensation
gas to a liquid
released 40.66
D4.13 Phase changes

 
The graph in image D4.14 shows the heating curve of water. It is important to note that every time the water changes phase, the graph becomes a horizontal line. This is because the temperature of water is not changing while it changes phases; instead, the thermal energy is either being released or absorbed as needed to break or form bonds.



D4.14 Heating curve of water

 

  Read This

Please read pages 383 to 385 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the new terms presented and the heating curve of water presented. Remember, if you have any questions or you do not understand something, ask your teacher!

  Practice Questions

Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

  1. What is the difference between the heat of vaporization and the heat of condensation?

    The heat of vaporization is a measure of the amount of thermal energy absorbed by 1 mol of a substance when it changes from a liquid to a gas; while the heat of condensation is the amount of thermal energy released while the substance changes from a gas to a liquid.

  2. What happens to the temperature of water as it changes phases? Why does this happen?

    The temperature does not change as water changes phases. This is because the thermal energy is being absorbed or released as part of the phase change, rather than being used to increase the temperature of the water. Another possible answer is the thermal energy is not being used to increase the kinetic energy of the particles during a phase change. Instead, the thermal energy is being absorbed or released by the forces between the particles, causing them to break or form.