Definition and Examples of Latent Heat

Specific latent heat (L) is defined as the amount of thermal energy (heat, Q) that is absorbed or released when a body undergoes a constant-temperature process. The equation for specific latent heat is:

L = Q / m

where:

• L is the specific latent heat
• Q is the heat absorbed or released
• m is the mass of a substance

The most common types of constant-temperature processes are phase changes, such as melting, freezing, vaporization, or condensation. The energy is considered to be "latent" because it is essentially hidden within the molecules until the phase change occurs. It is "specific" because it is expressed in terms of energy per unit mass. The most common units of specific latent heat are joules per gram (J/g) and kilojoules per kilogram (kJ/kg).

Specific latent heat is an intensive property of matter. Its value does not depend on sample size or where within a substance the sample is taken.

History

British chemist Joseph Black introduced the concept of latent heat somewhere between the years of 1750 and 1762. Scotch whiskey makers had hired Black to determine the best mixture of fuel and water for distillation and to study changes in volume and pressure at a constant temperature. Black applied calorimetry for his study and recorded latent heat values.

English physicist James Prescott Joule described latent heat as a form of potential energy. Joule believed the energy depended on the specific configuration of particles in a substance. In fact, it is the orientation of atoms within a molecule, their chemical bonding, and their polarity that affect latent heat.

Types of Latent Heat Transfer

Latent heat and sensible heat are two types of heat transfer between an object and its environment. Tables are compiled for the latent heat of fusion and latent heat of vaporization. Sensible heat, in turn, depends on the composition of a body.

• Latent Heat of Fusion: Latent heat of fusion is the heat absorbed or released when matter melts, changing phase from solid to liquid form at a constant temperature.
• Latent Heat of Vaporization: The latent heat of vaporization is the heat absorbed or released when matter vaporizes, changing phase from liquid to gas phase at a constant temperature.
• Sensible Heat: Although sensible heat is often called latent heat, it isn't a constant-temperature situation, nor is a phase change involved. Sensible heat reflects heat transfer between matter and its surroundings. It is the heat that can be "sensed" as a change in an object's temperature.

Table of Specific Latent Heat Values

This is a table of specific latent heat (SLH) of fusion and vaporization for common materials. Note the extremely high values for ammonia and water compared to that of nonpolar molecules.

Sensible Heat and Meteorology

While latent heat of fusion and vaporization are used in physics and chemistry, meteorologists also consider sensible heat. When latent heat is absorbed or released, it produces instability in the atmosphere, potentially producing severe weather. The change in latent heat alters the temperature of objects as they come into contact with warmer or cooler air. Both latent and sensible heat cause air to move, producing wind and vertical motion of air masses.

Examples of Latent and Sensible Heat

Daily life is filled with examples of latent and sensible heat:

• Boiling water on a stove occurs when thermal energy from the heating element is transferred to the pot and in turn to the water. When enough energy is supplied, liquid water expands to form water vapor and the water boils. An enormous amount of energy is released when water boils. Because water has such a high heat of vaporization, it's easy to get burned by steam.
• Similarly, considerable energy must be absorbed to convert liquid water to ice in a freezer. The freezer removes thermal energy, allowing the phase transition to occur. Water has a high latent heat of fusion, so turning water into ice requires the removal of more energy than freezing liquid oxygen into solid oxygen, per unit gram.
• Latent heat causes hurricanes to intensify. Air heats as it crosses warm water and picks up water vapor. As the vapor condenses to form clouds, latent heat is released into the atmosphere. This added heat warms the air, producing instability and helping clouds to rise and the storm to intensify.
• Sensible heat is released when soil absorbs energy from sunlight and gets warmer.
• Cooling via perspiration is affected by latent and sensible heat. When there is a breeze, evaporative cooling is highly effective. Heat is dissipated away from the body due to the high latent heat of vaporization of water. However, it's much harder to cool down in a sunny location than in a shady one because sensible heat from absorbed sunlight competes with the effect from evaporation.

Sources

• Bryan, G.H. (1907). Thermodynamics. An Introductory Treatise Dealing Mainly With First Principles and Their Direct Applications. B.G. Teubner, Leipzig.
• Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8.
• Maxwell, J.C. (1872). Theory of Heat, third edition. Longmans, Green, and Co., London, page 73.
• Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.