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Latent Heat: Denition, Formula & ExamplesChapter 6 (/academy/topic/ged-science-energy-and-heat-help-and-review.html) / Lesson 30(/academy/lesson/latent-heat-denition-formula-examples.html#transcriptHeader)

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Instructor: Amy Lange

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

Heat is an important component of phase changes. This lesson will delve into the concept of latentheat and how it aects of the behavior of matter during phase changes.

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

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If we measured the temperature of this boiling water, it would stay constant at 100 degrees Celsius until all of the water hadvaporized.

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

Denition of Latent HeatNormally when heat energy is added to or removed from an object, the temperature ofthe object changes; however during phase changes, the temperature of an object staysconstant. The temperature remains the same because energy is required for an objectto change phases.

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

FormulaThe equation for latent heat is:

Q = m * L



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

The values of latent heat are variable depending on the nature of the phase changetaking place. The latent heat of fusion is the change from liquid to solid, the latentheat of vaporization is from liquid to gas, the latent heat of sublimation is thechange from solid to gas.

Examples of Latent HeatIn the previous example of boiling water, we know that we much continue to add energyto the water before all of the liquid water turns to steam. The latent heat of vaporizationfor water is 22.6 x 10^5 J/kg. This means that 22.6 x 10^5 J of heat energy must be addedto 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 blueboxes, the temperature does not rise despite additional energy being added. This added energy is going toward overcomingthe latent heat of the water.



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latent heat_ definition, formula & examples _ study

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