Latent heat 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. latent heat. The quantity of heat absorbed or released by a substance undergoing a change of state, such as ice changing to water or water to steam, at constant temperature and pressure. Also called heat of transformation. Noun 1. latent heat - heat absorbed or radiated during a change of phase at a constant temperature and pressure heat of transformation heat, heat energy - a form of energy that is transferred by a difference in temperature heat of condensation - heat liberated by a unit mass of gas at its boiling point as it condenses into a liquid; "the heat of condensation is equal to the heat of vaporization" heat of fusion - heat absorbed by a unit mass of a solid at its melting point in order to convert the solid into a liquid at the same temperature; "the heat of fusion is equal to the heat of solidification" heat of solidification - heat liberated by a unit mass of liquid at its freezing point when it solidifies heat of sublimation - heat absorbed by a unit mass of material when it changes from a solid to a gaseous state heat of vaporisation, heat of vaporization - heat absorbed by a unit mass of a material at its boiling point in order to convert the material into a gas at the same temperature 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 whisky 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 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 constant temperature. Latent Heat of Vaporization: Latent heat of vaporization is the heat absorbed or released when matter vaporizes, changing phase from liquid to gas phase at 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. Material Melting Point (°C) Boiling Point (°C) SLH of Fusion kJ/kg SLH of Vaporization kJ/kg Ammonia −77.74 −33.34 332.17 1369 Carbon Dioxide −78 −57 184 574 Ethyl Alcohol −114 78.3 108 855 Hydrogen −259 −253 58 455 Lead 327.5 1750 23.0 871 Nitrogen −210 −196 25.7 200 Oxygen −219 −183 13.9 213 Refrigerant R134A −101 −26.6 — 215.9 Toluene −93 110.6 72.1 351 Water 0 100 334 2264.705 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 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. Tuebner, Leipzig. Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-76074616-8. 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.
