Latent Heat

Latent heat is energy absorbed or released by a substance during a change in its physical state (phase) that occurs without changing its temperature.[2] The heat is released or absorbed by a body or a thermodynamic system during a constant-temperature process.[1] Latent heat arises from the work required to overcome the forces that hold together atoms or molecules in a material.[2]

Definition and Physical Principles

Latent heat is the amount of heat energy that a substance absorbs or releases during a change of state (phase change) without any change in its temperature.[6] When a substance changes from one state to another, such as solid to liquid or liquid to gas, heat is required to break the forces between its particles. This heat does not increase the temperature, so it is called latent heat, meaning "hidden heat."[6]

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.[2]

Types of Latent Heat

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.[2]

Latent Heat of Fusion

The latent heat that is associated with the melting of a solid or freezing of a liquid is called the heat of fusion.[1] The heat of fusion for water at 0 °C is approximately 334 joules (79.7 calories) per gram.[2] 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.[2]

Latent Heat of Vaporization

The latent heat that is associated with the vaporizing of a liquid or a solid or condensing vapor is called the heat of vaporization.[1] The heat of vaporization at 100 °C is about 2,230 joules (533 calories) per gram.[2] 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.[2]

Latent Heat of Sublimation

Latent heat of sublimation is the amount of energy in the form of heat that is needed to convert 1 kg of a solid substance to vapor at the same temperature (and not by means of passing through a liquid state).[6] When the sublimation process occurs, the temperature of the substance does not change although heat is added continuously. The heat energy supplied is utilized in overcoming the strong intermolecular forces existing in the solid to allow the particles to move freely like gas particles. This is the heat energy that is absorbed when the process of sublimation is taking place, and this heat energy is referred to as the latent heat of sublimation.[6]

Mathematical Expression

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.[2] The latent heat formula, Q = m * L, quantifies this energy exchange, where Q is the heat energy, m is the mass, and L is the latent heat.[7]

Applications and Significance

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.[2] Water has a high latent heat of vaporization, which is why steam burns are so dangerous. When steam burns a person's arm for example, this energy transfer causes the steam to condense—which uses much more energy than simply changing the temperature.[4]

Understanding latent heat is essential in meteorology and climate science. The release of latent heat during condensation plays a vital role in cloud formation, precipitation, and weather patterns.[7] In engineering applications, knowing the latent heat of substances is crucial for designing and operating systems that involve phase changes, such as refrigeration, air conditioning, and heat exchangers.[7]

James Watt and Latent Heat

James Watt met many scientists and became a friend of British chemist and physicist Joseph Black, who developed the concept of latent heat.[11] Watt had realized that the loss of latent heat (the heat involved in changing the state of a substance—e.g., solid or liquid) was the worst defect of the Newcomen engine and that therefore condensation must be effected in a chamber distinct from the cylinder but connected to it.[11]

While at the University of Glasgow he attended the lectures of Joseph Black, who was developing his theory of latent heat, and he also became well acquainted with John Robison, a brilliant young chemist. Robison directed his attention to the problems of existing steam engines, and when Watt was asked, during the winter of 1763-1764, to repair a model of the Newcomen engine, he was led to a critical study of its workings.[12]

He learned much about steam properties, and independently discovered latent heat of vaporization in his experiments.[13] One of his University friends was Professor Black, who had discovered latent heat previously and had been lecturing on it without Watt's knowledge. They shared many interesting conversations after Watt told Professor Black of his "discovery".

Black had just found out about latent heat. He showed that water absorbs a huge lot of energy, without changing temperature, when you boil it.[16] So Black answered that his result was because of latent heat. Later, Watt wrote, Thus I stumbled upon one of the material facts by which [Black's] beautiful theory is supported.

Watt realized that, to save fuel required for constantly reheating the cooled Newcomen cylinder, the steam should be condensed away from the cylinder, allowing it to remain hot. Black was able to help Watt apply the former's concept of latent heat to the problem of keeping the cylinder hot in the atmospheric engine. This "separate condenser," invented in 1765, was the first improvement on Newcomen's engine, and it saved three-fourths of the fuel.[12]

Historical Development

Joseph Black (1728-1799) was a British chemist and physicist best known for the rediscovery of "fixed air" (carbon dioxide), the concept of latent heat, and the discovery of the bicarbonates.[21] Black distinguished between the quantity of heat in a body and its intensity, or temperature, realizing that thermometers can be used to determine the quantity of heat if temperature is measured over a period of time while the body is heated or cooled. He took two similar glass flasks, pouring the same quantity of water into both and placing them in a freezing mixture. In one he had added a little alcohol to prevent freezing. They were then removed from the bath, one frozen and the other liquid, though at the same temperature. They were allowed to warm up naturally. The temperature of the water plus alcohol warmed up several degrees, while the ice remained at its freezing point. As the flasks had to be absorbing heat at the same rate, Black showed that the heat absorbed by the ice in 10 hours would have raised the temperature of the same quantity of water by 78 °C (140 °F). This was the latent heat of fusion of water.[21]

In both cases, considered as the cause of warmth, we do not perceive its presence: it is concealed, or latent, and I give it the name of LATENT HEAT.[27] The Scottish scientific expert, Joseph Black, presented the idea of latent heat somewhere close to the period 1750 and 1762. Scotch bourbon producers had employed Black to decide the best blend of fuel and water for refining and to examine changes in volume and weight at a steady temperature. Dark applied calorimetry for his investigation and recorded latent heat esteems.[3]

Extended Applications

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.[2]

The latent heat storage can be easily achieved in buildings through solid-liquid phase transition.[1] Materials with high latent heat, such as paraffin wax and Glauber's salt, are used in phase change thermal storage systems. They offer high energy storage capacity with minimal temperature variation.[9]