Latent heat Most substances can exist in three states: gas, liquid and solid, though there is an additional

state named plasma. The main difference between a substance in each state is how quickly its molecules are moving. As a liquid, the molecules move at a speed where they can repeatedly join together, break apart, and then join together again. When they move slowly, they stay joined together, forming a solid. When they move quickly, they stay broken apart, forming a gas. For example, we usually think of water as a liquid. However, it can also be a solid (ice) or a gas (steam). But as you can see when you boil water in a kettle or when the surface of a pond freezes, not all of the molecules in a substance change state at the same time. When a molecule changes state, it has a different amount of energy. However, the laws of physics state that energy cant just disappear. So when the molecule moves more slowly, the excess energy is released into the surroundings as latent heat. When the molecule moves more quickly, it has absorbed extra energy by taking latent heat from the surroundings. Latent heat is also the specific enthalpy difference between two phases of a substance at the same temperature. In thermochemistry, it is the heat released or absorbed by a body during a change of state without change of temperature. The term most often refers to a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1750 by Joseph Black. It is derived from the Latin latere (to lie hidden). In its original context of calorimetry set by Black, besides phase changes, the term referred in particular to the heat transferred to a body upon change of volume at constant temperature without phase change. It is also the characteristic amount of energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature. 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 vapor 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 (80 calories) per gram, and the heat of vaporization at 100 C is about 2,260 joules (540 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 vapor 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 vapor 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. The term latent heat was introduced into calorimetry around 1750 by Joseph Black. James Prescott Joule characterized latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy that was indicated by the thermometer, relating the latter to thermal energy.

JOSEPH BLACK

Joseph Black FRSE FRCPE FPSG (16 April 1728 6 December 1799) was a Scottish physician and chemist, known for his discoveries of latent heat, specific heat, and carbon dioxide. He was professor of Medicine at University of Glasgow (where he also served as lecturer in Chemistry). James Watt, who was appointed as philosophical instrument maker at the same university (1756). Watt consulted with Black on experiments with his steam engine. Watt and Black also collaborated in project to manufacture sodium hydroxide; however, Black was not known to have any business interest in the process, which did not enjoy commercial success. The chemistry buildings at both the Edinburgh and the University of Glasgow are named after Black. In 1761 Black deduced that the application of heat to ice at its melting point does not cause a rise in temperature of the ice/water mixture, but rather an increase in the amount of water in the mixture. Additionally, Black observed that the application of heat to boiling water does not result in a rise in temperature of a water/steam mixture, but

rather an increase in the amount of steam. From these observations, he concluded that the heat applied must have combined with the ice particles and boiling water and become latent. The theory of latent heat marks the beginning of thermodynamics. Black's theory of latent heat was one of his more-important scientific contributions, and one on which his scientific fame chiefly rests. He also showed that different substances have different specific heats. This all proved important not only in the development of abstract science but in the development of the steam engine. The latent heat of water is large compared with many other liquids, so giving impetus to James's successful attempts to improve the efficiency of the steam engine invented by Thomas Newcomen. Watt added a separate condenser, and kept the cylinder at the temperature of steam (by enclosing it in a steam-filled jacket) so saving a considerable amount of energy in avoiding the reheating of the cylinder at every cycle of the engine.

The worlds first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes, calculations which were based on Joseph Blacks prior discovery of latent.

2 FORMS OF LATENT HEAT

The enthalpy of fusion also known as the latent heat of fusion is the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point. Latent heat of fusion is the amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature. It is a latent heat because during melting the introduction of heat cannot be observed as a temperature change, as the temperature remains constant during the process. The latent heat of fusion is the enthalpy change of any amount of substance when it melts. When the heat of fusion is referenced to a unit of mass, it is usually called the specific heat of fusion, while the molar heat of fusion refers to the enthalpy change per amount of substance in moles.

The liquid phase has a higher internal energy than the solid phase. This means energy must be supplied to a solid in order to melt it and energy is released from a liquid when it freezes, because the molecules in the liquid experience weaker intermolecular forces and have a larger potential energy. When liquid water is cooled, its temperature falls steadily until it drops just below the freezing point at 0 C. The temperature then remains constant at the freezing point while the water crystallizes. Once the water is completely frozen, its temperature continues to fall. The enthalpy of fusion is almost always a positive quantity; helium is the only known exception. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat.

2. Latent heat of vaporization

The enthalpy of vaporization, (symbolHvap), also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas at a given pressure (often atmospheric pressure). It is the amount of heat required to convert a unit mass of a liquid at its boiling point into vapor without an increase in temperature. It is often measured at the normal boiling point of a substance; although tabulated values are usually corrected to 298 K, the correction is often smaller than the uncertainty in the measured value. The heat of vaporization is temperature-dependent, though a constant heat of vaporization can be assumed for small temperature ranges and below Tr<<1.0. The heat of vaporization diminishes with increasing temperature and it vanishes completely at the critical temperature (Tr=1) because above the critical temperature the liquid and vapor phases no longer co-exist.