What is latent heat?

Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process.

An example is a state of matter change, meaning a phase transition, such as ice melting or water boiling.

The term was introduced around 1762 by Scottish chemist Joseph Black. It is derived from the Latin latere (to lie hidden). Black used the term in the context of calorimetry where a heat transfer caused a volume change while the thermodynamic system's temperature was constant.

2 Replies to “What is latent heat?”

  1. 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 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 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.

    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.
  2. Latent heat is the amount of heat absorbed or released when a substance changes phase (freezes or vaporizes). It is called “latent” because there is no temperature change involved even though there is a heat effect. The other kind of heat is called “sensible heat” which involves a temperature change.

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