Introduction the Latent Heat

The word “latent” has come from the Latin word “latere" which means to hide or cover up. This explains the concept of latent heat which is the extra heat required to change the state of a substance, for example, if a substance is in the solid-state, latent heat provides the extra energy to change its state to liquid and then from the liquid state to the gaseous state. This heat is required apart from the heat given to melt or vaporize the substance and it works without raising or decreasing the actual temperature of the substance no matter which state it is currently in.

Latent Heat of Fusion

The latent heat of fusion or melting, of a solid, is the quantity of heat in joules required to transform a solid, at its melting point, to a liquid without any variation in temperature. The latent heat of fusion of ice is 3.34 x 10⁵ joules per kilograms (or, 3.34 x 10⁵J/Kg).

The enthalpy change during melting or fusion is the latent heat that measures the amount of substance dissolved when transforming from the state of solid to the liquid state. The liquid state of a substance requires higher inward energy to convert to the solid-state which means energy must be provided to the solid-state to convert the substance into its fluid state, therefore, when the substance converts back to its solid-state, it releases energy in the form of latent heat.

Therefore, the enthalpy of melting is, mostly, a positive quantity with the exception of Helium-3.

Latent Heat of Vaporization

The term latent heat of vaporization is the amount of heat required for the transformation of a liquid at its boiling point to gas at a constant temperature. The latent heat of vaporization differs for various liquids. 

In the figure given above, the liquid is converting into vapour by heating.Particles of water vapour at 100°C (373K) have more energy than liquid water at the same temperature. This is due to the assimilation of extra energy in the form of latent heat of vaporization.

The heat of vaporization is characterized as the amount of energy (heat) required to convert 1g of a liquid into a gas without any change in the temperature of the liquid itself. Therefore, like the heat of fusion, the latent heat of vaporization measures the heat given to a liquid in order to change its state into its gaseous state. In case of water, the latent heat to transform it into vapour is the energy of boiling water at which it changes form without change in temperature. This latent heat amounts to 22.5×105 J/kg.

FAQs on Latent Heat

What is meant by the term “latent heat”?

Latent heat is the energy required to change the state of a substance from solid to liquid and vice-versa as well as from liquid to gas and vice-versa, and from the solid state to the gaseous state and vice-versa without changing the temperature of the substance.

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What are the types of latent heat and what are they?

There are three types of latent heat based on the three states in which a substance can exist, i.e., solid liquid and gas, therefore, the latent heat accordingly are:

  • Latent heat of fusion: the heat or energy required to change the substance from solid to liquid state or back from liquid to solid state.

  • Latent heat of vaporization: the heat required to convert the substance from liquid to gas and vice-versa.

  • Latent heat of sublimation: the amount of heat or energy needed to transform a solid substance to its gaseous state and back to its original state.

What are the values of latent heat of fusion of ice and the latent heat of vaporization of water?

The latent heat of fusion of ice is 334 J/kg at 0°C, and the latent heat of water is 22.5×105 J/kg.

What are some practical examples of latent heat?

Some instances where latent heat is responsible:

  • Change in latent heat causes the air around us to become warmer or cooler.

  • Latent heat causes air to move around and cause wind and vertical movements of air.

  • Camphor and naphthalene sublime at normal room temperature diminishing into thin air.

  • Boiling hot water creates water bubbles that burst and release the heat which when comes in skin contact, can cause severe burns.

  • A refrigerator can convert water to the ice by providing the necessary heat requirements.

Where are some applications of latent heat?

Some applications of latent heat in real life:

  • Cocoa butter is a kind of fat that has a regular composition, unlike most other fats which are some kind of mixtures. Therefore, when cocoa butter melts it absorbs heat at a definite melting point. This helps make chocolates cool.

  • During combustion, the condensation furnace comes with a heat exchanger which is used to extract heat from the exhaust gases and helps in the condensation of water, thereby, trapping its latent heat.

  • People who grow oranges, use ice to keep them from freezing.

  • During roasting meat, the interiors never exceed temperatures higher than the boiling point of water. If that happens then there would be no water left in the meat which would make it have a leathery.