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October 4, 2022

Latent & Sensible Heat

Heat is the form of energy that flows or transfers from one object to another due to their temperature difference to establish the thermal balance. It can be categorized as sensible heat and latent heat. Sensible heat relates to the term “sense”. Therefore, it is the form of heat that can be felt or sensed due to the change of temperature with no change in pressure or volume. Latent heat, in contrast, is the “hidden” heat that is the energy absorbed or released by a substance resulting in the phase change at a fixed temperature which could be melting or boiling point or vice versa for the release of heat.

During the latent phase, the thermal energy supplied/ released is used to overcome the intermolecular forces of the substance that are responsible to maintain its state. This results in a phase change of the object. The latent heat of crystallization is released when water turns into ice. During this exothermic reaction, 1g of water when frozen into 1g of ice releases 80 calories of heat. The heat released tends to make the water molecules more orderly and attain spatial symmetry. They slow down and eventually stick to each other to become denser than the ice making a liquid-liquid phase transition.

An unusual phenomenon observed in water is a negative thermal expansion which means water expands instead of contracting on freezing, making it less viscous on solidifying. Based on the arrangement of water molecules upon freezing, ice exists in 17 crystalline phases. One of the interesting applications based on the concept/ exploitation of latent heat is the presence of high-density water. The phase transition behaviour of water to maintain a high-density liquid is due to the co-occurrence of two processes. Firstly, the high density is due to the shortening of hydrogen bonds that allows more molecules within the given volume, and secondly continuation of asymmetrical arrangement. This interplay between bond orientation and density forms the basis of supercooled water.

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