Latent heat is the energy a substance absorbs or releases during a change in its physical state (or phase) without changing its temperature. It is usually expressed as the amount of heat (in joules or calories) per unit mass of the substance. Latent heat is associated with changes among solid, liquid, and vapor phases, and many solids exist in different crystalline forms with transitions that involve latent heat. Solar radiation converts to sensible and latent heat on the Earth's surface. A change in sensible heat changes the temperature of a medium, while latent heat drives a process, such as a phase change from liquid to gas, without changing the temperature. The latent heat associated with melting or freezing is called the heat of fusion, while that associated with vaporizing or condensing is called the heat of vaporization. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses.

When does latent heat play a role in changes of state, like melting or boiling?

Latent heat is the energy released or absorbed by a substance during a change in its physical state that happens without changing its temperature. It plays a role during phase transitions such as melting or freezing (heat of fusion), and vaporization or condensation (heat of vaporization). For instance, when water boils, its temperature remains constant at 100 °C until all the water evaporates because the heat is absorbed as latent heat of vaporization. Similarly, when ice melts, it remains at 0 °C, with the latent heat of fusion contributing to the formation of liquid water, also at 0 °C. Latent heat arises from the energy required to overcome the forces that hold together atoms or molecules in a material.

What is the difference between latent heat of fusion and latent heat of vaporization?

Latent heat is energy released or absorbed by a substance during a change in its physical state that happens without changing its temperature. Latent heat of fusion is associated with melting a solid or freezing a liquid. For water at 0 °C, the heat of fusion is about 334 joules per gram. Latent heat of vaporization is associated with vaporizing a liquid or solid or condensing a vapor. The heat of vaporization for water at 100 °C is about 2,230 joules per gram. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses, making water an excellent working fluid for heat engines. Latent heat arises from the work required to overcome the forces that hold together atoms or molecules in a material.

Why doesn’t the temperature change during the process of melting or boiling, even when heat is added?

During melting or boiling, the temperature remains constant despite the addition of heat because the energy is used to overcome the forces holding the atoms or molecules together. This energy is known as latent heat , and it's absorbed or released during a phase change without changing the temperature. For instance, when water boils, the temperature stays at 100 °C (212 °F) until all the water evaporates. The added heat is absorbed as latent heat of vaporization, which is then carried away by the escaping vapor molecules. Similarly, when ice melts, it remains at 0 °C (32 °F), with the latent heat of fusion being used to convert it into liquid water at the same temperature.

How does latent heat help explain why ice is used to keep drinks cold?

Latent heat explains why ice is effective for cooling drinks. As ice melts, it absorbs energy in the form of heat from its surroundings without changing temperature; this is called the latent heat of fusion . For water, the heat of fusion is approximately 334 joules per gram. The ice absorbs heat from the drink, which lowers the drink's temperature, providing a cooling effect. Because the ice melts instead of immediately warming up, it can keep the drink cold longer.

What is a real-life example where understanding latent heat is important, such as in weather or cooking?

Latent heat is crucial in many real-world scenarios. For instance, when water boils in a pot, the temperature remains constant at 100°C until all the water evaporates. This happens because the added heat is absorbed as latent heat of vaporization , providing the energy needed for the water to change from liquid to gas. A practical application of understanding latent heat is in refrigeration , where the rapid expansion of gas cools the surroundings. Also, the evaporation of surface moisture, which produces atmospheric water vapour, has a cooling effect. Most of the latent heat contained in water vapour is subsequently released to the atmosphere during the formation of precipitating clouds.

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What is latent heat?
Latent heat is the energy a substance absorbs or releases during a change in its physical state (or phase) without changing its temperature. It is usually expressed as the amount of heat (in joules or calories) per unit mass of the substance.

Latent heat is associated with changes among solid, liquid, and vapor phases, and many solids exist in different crystalline forms with transitions that involve latent heat. Solar radiation converts to sensible and latent heat on the Earth's surface. A change in sensible heat changes the temperature of a medium, while latent heat drives a process, such as a phase change from liquid to gas, without changing the temperature.

The latent heat associated with melting or freezing is called the heat of fusion, while that associated with vaporizing or condensing is called the heat of vaporization. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses.
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When does latent heat play a role in changes of state, like melting or boiling?
Latent heat is the energy released or absorbed by a substance during a change in its physical state that happens without changing its temperature. It plays a role during phase transitions such as melting or freezing (heat of fusion), and vaporization or condensation (heat of vaporization).

For instance, when water boils, its temperature remains constant at 100 °C until all the water evaporates because the heat is absorbed as latent heat of vaporization. Similarly, when ice melts, it remains at 0 °C, with the latent heat of fusion contributing to the formation of liquid water, also at 0 °C. Latent heat arises from the energy required to overcome the forces that hold together atoms or molecules in a material.
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What is the difference between latent heat of fusion and latent heat of vaporization?
Latent heat is energy released or absorbed by a substance during a change in its physical state that happens without changing its temperature.
- Latent heat of fusion is associated with melting a solid or freezing a liquid. For water at 0 °C, the heat of fusion is about 334 joules per gram.
- Latent heat of vaporization is associated with vaporizing a liquid or solid or condensing a vapor. The heat of vaporization for water at 100 °C is about 2,230 joules per gram. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses, making water an excellent working fluid for heat engines.
Latent heat arises from the work required to overcome the forces that hold together atoms or molecules in a material.
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2. latent heat
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Why doesn’t the temperature change during the process of melting or boiling, even when heat is added?
During melting or boiling, the temperature remains constant despite the addition of heat because the energy is used to overcome the forces holding the atoms or molecules together. This energy is known as latent heat, and it's absorbed or released during a phase change without changing the temperature.

For instance, when water boils, the temperature stays at 100 °C (212 °F) until all the water evaporates. The added heat is absorbed as latent heat of vaporization, which is then carried away by the escaping vapor molecules. Similarly, when ice melts, it remains at 0 °C (32 °F), with the latent heat of fusion being used to convert it into liquid water at the same temperature.
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How does latent heat help explain why ice is used to keep drinks cold?
Latent heat explains why ice is effective for cooling drinks. As ice melts, it absorbs energy in the form of heat from its surroundings without changing temperature; this is called the latent heat of fusion. For water, the heat of fusion is approximately 334 joules per gram.

The ice absorbs heat from the drink, which lowers the drink's temperature, providing a cooling effect. Because the ice melts instead of immediately warming up, it can keep the drink cold longer.
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1. latent heat
2. ice: Thermal properties
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What is a real-life example where understanding latent heat is important, such as in weather or cooking?
Latent heat is crucial in many real-world scenarios. For instance, when water boils in a pot, the temperature remains constant at 100°C until all the water evaporates. This happens because the added heat is absorbed as latent heat of vaporization, providing the energy needed for the water to change from liquid to gas.

A practical application of understanding latent heat is in refrigeration, where the rapid expansion of gas cools the surroundings. Also, the evaporation of surface moisture, which produces atmospheric water vapour, has a cooling effect. Most of the latent heat contained in water vapour is subsequently released to the atmosphere during the formation of precipitating clouds.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

latent heat, energy absorbed or released by a substance during a change in its physical state (phase) that occurs without changing its temperature. The latent heat associated with melting a solid or freezing a liquid is called the heat of fusion; that associated with vaporizing a liquid or a solid or condensing a vapour is called the heat of vaporization. The latent heat is normally expressed as the amount of heat (in units of joules or calories) per mole or unit mass of the substance undergoing a change of state.

For example, when a pot of water is kept boiling, the temperature remains at 100 °C (212 °F) until the last drop evaporates, because all the heat being added to the liquid is absorbed as latent heat of vaporization and carried away by the escaping vapour molecules. Similarly, while ice melts, it remains at 0 °C (32 °F), and the liquid water that is formed with the latent heat of fusion is also at 0 °C. The heat of fusion for water at 0 °C is approximately 334 joules (79.7 calories) per gram, and the heat of vaporization at 100 °C is about 2,230 joules (533 calories) per gram. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses, making water an excellent working fluid for heat engines.

Latent heat arises from the work required to overcome the forces that hold together atoms or molecules in a material. The regular structure of a crystalline solid is maintained by forces of attraction among its individual atoms, which oscillate slightly about their average positions in the crystal lattice. As the temperature increases, these motions become increasingly violent until, at the melting point, the attractive forces are no longer sufficient to maintain the stability of the crystal lattice. However, additional heat (the latent heat of fusion) must be added (at constant temperature) in order to accomplish the transition to the even more-disordered liquid state, in which the individual particles are no longer held in fixed lattice positions but are free to move about through the liquid. A liquid differs from a gas in that the forces of attraction between the particles are still sufficient to maintain a long-range order that endows the liquid with a degree of cohesion. As the temperature further increases, a second transition point (the boiling point) is reached where the long-range order becomes unstable relative to the largely independent motions of the particles in the much larger volume occupied by a vapour or gas. Once again, additional heat (the latent heat of vaporization) must be added to break the long-range order of the liquid and accomplish the transition to the largely disordered gaseous state.

Latent heat is associated with processes other than changes among the solid, liquid, and vapour phases of a single substance. Many solids exist in different crystalline modifications, and the transitions between these generally involve absorption or evolution of latent heat. The process of dissolving one substance in another often involves heat; if the solution process is a strictly physical change, the heat is a latent heat. Sometimes, however, the process is accompanied by a chemical change, and part of the heat is that associated with the chemical reaction. See also melting.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Amy McKenna.

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