3.2.3 Changes of State and Specific Latent Heat

In this lesson, we will explore the energy involved in changes of state and the concept of specific latent heat. We will learn how energy is supplied to change the state of a substance without altering its temperature and understand the difference between specific heat capacityAmount of heat energy required to increase the temperature of one unit of mass of a substance by one unit of temperature. and specific latent heat.

Changes of State and Latent Heat

Changes of state refer to the physical transformations that occur when a substance transitions between different states, such as solid, liquid, and gas.

Latent heat is the energy required for a substance to undergo a change of state without a change in temperature. During a change of state, the energy supplied or released changes the internal energyTotal kinetic and potential energy of all particles within a system. of the substance but does not cause a temperature change.

Specific Latent Heat

Specific latent heat refers to the amount of energy required to change the state of one kilogram of a substance without a change in temperature.

• Specific Latent Heat of Fusion: The specific latent heat of fusion is the amount of energy required to change one kilogram of a substance from a solid to a liquid state.
• Specific Latent Heat of Vaporisation: The specific latent heat of vaporisation is the amount of energy required to change one kilogram of a substance from a liquid to a vapour (gas) state.

The energy (E) required for a change of state is calculated using the equation:

E = m * L

Variables:

• E = Energy in joules, J
• m = Mass of the substance undergoing the change of state in kilograms, kg
• L = Specific latent heat of the substance in joules per kilogram, J/kg

Heating and Cooling Graphs

Let's discuss how to interpret heating and cooling graphs that include changes of state.

Heating Graphs:

• Plateaus: During changes of state, the temperature remains constant as energy is absorbed or released. This results in flat sections, or plateaus, on the heating graph.
• Identification: The plateaus on the graph indicate the specific latent heat regions, where the substance is undergoing a change of state.

Cooling Graphs:

• Plateaus: Similarly, during changes of state, the temperature remains constant as energy is absorbed or released. This results in flat sections, or plateaus, on the cooling graph.
• Identification: The plateaus on the graph indicate the specific latent heat regions, where the substance is undergoing a change of state.

Specific Heat Capacity vs. Specific Latent Heat

Let's differentiate between specific heat capacity and specific latent heat.

• Specific Heat Capacity: Specific heat capacity is the amount of energy required to raise the temperature of a substance by one degree Celsius per unit mass.It is measured in joules per kilogram per degree Celsius (J/kg °C). Specific heat capacity is associated with temperature changes within the same state of matter.
• Specific Latent Heat: Specific latent heat is the amount of energy required to change the state of a substance without a change in temperature. It is measured in joules per kilogram (J/kg). Specific latent heat is associated with changes of state, such as melting, freezing, boiling, or condensing.

Conclusion

We explored changes of state and specific latent heat. Changes of state involve the energy required for a substance to transition between different states without a change in temperature. The specific latent heat represents the amount of energy required to change the state of one kilogram of a substance. Specific latent heat differs from specific heat capacity, which is associated with temperature changes within the same state of matter.