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Latent Heat: Definition, Formula & Examples

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Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

Latent heat is a type of heat energy, described in energy per unit of mass, that must be achieved for a phase change to occur in a substance. See this concept's formula and how it is used in a set of examples of phases: fusion (solidifying), vaporization (liquid to gas), and sublimation (solid to gas). Updated: 10/26/2021

Phase Changes

Boiling a pot of water is not as simple as just turning on the stove. As you apply heat to the water, the temperature increases until it reaches 100 degrees Celsius, the boiling point of water. The water will stay at this temperature until all of the water changes from liquid to a gas. During this process, you are continuing to add heat energy to the water, but the water temperature does not increase. Where is this energy going? The answer lies in the concept of latent heat.

To understand this concept of latent heat, we must first review phase changes. Phase changes refer to a change in matter from one state to another. The most familiar phase changes are seen in the different states of water, such as freezing liquid water to create ice or boiling liquid water to create a gas. As you can tell, heat plays a major role in changing matter from one phase to another.

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  • 0:55 Definition of Latent Heat
  • 1:40 Formula for Latent Heat
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Definition of Latent Heat

Normally, when heat energy is added to or removed from an object, the temperature of the object changes; however, during phase changes, the temperature of an object stays constant. The temperature remains the same because energy is required for an object to change phases.

Latent heat is the heat energy per mass unit required for a phase change to occur. If we think about substances at a molecular level, gaseous molecules have more vibration than liquid molecules. So when you add heat to a liquid, you are actually causing the molecules to vibrate. The latent heat is the energy required to change the molecular movement. Each substance has a unique latent heat value.

Formula for Latent Heat

The formula for latent heat is:

Q = m * L

This equation relates the heat Q that must be added or removed for an object of mass m to change phases. The object's individual latent heat is noted by L. The unit of latent heat is J/kg.

The values of latent heat are variable depending on the nature of the phase change taking place:

  • The latent heat of fusion is the change from liquid to solid.
  • The latent heat of vaporization is from liquid to gas.
  • The latent heat of sublimation is the change from solid to gas.

Examples of Latent Heat

In the previous example of boiling water, we know that we must continue to add heat energy to the water before all of the liquid water turns to steam. The latent heat of vaporization for water is 22.6 x 10^5 J/kg. This means that 22.6 x 10^5 J of heat energy must be added to turn one kilogram of water from liquid to gas at 100 degrees Celsius.

This graph shows how the temperature of water changes with added heat. During the phase changes, marked by the blue boxes, the temperature does not rise despite additional energy being added. This added energy is going toward overcoming the latent heat of the water.
Graph showing the change of temperature with added energy in water

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