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Latent Heat

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When a solid is heated, its temperature rises due to the increase of the internal energy of the solid. The increased internal energy is in the form of increased kinetic energy of the molecule in the solid. They vibrate more when the solid is heated and its temperature rises. Continued heating increase the temperature to the melting point. At the melting point, further heating makes the solid liquify without change of temperature. Once it is completely liquified, the temperature continues to rise if more energy is supplied. To change the state of the material from solid to liquid, energy must be supplied to the material. Energy transferred to or form a material as a result of change of state is called latent heat. Latent heat must be supplied for changes from 
  1. solid to liquid 
  2. liquid to gas 
  3. solid to gas 
The heat energy present during the changes of state, which does not manifest itself externally by any rise or fall in temperature is called Latent Heat.

When we express latent heat in terms of unit mass of a substance, it is called Specific Latent Heat.

Latent Heat Formula

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To change the state of mass $m$ of material, the heat energy required is given by

$Heat\ Required$ = $Latent\ heat \times Mass\ of\ the\ substance$

$Q = L \times m$

$Latent\ heat$ = $\frac{Heat\ required}{Mass\ of\ the\ substance}$

$L$ = $\frac{Q}{m}$ J/kg

Latent Heat of Fusion

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The quantity of heat required per unit mass of a solid substance to completely convert it into liquid at constant temperature (i.e. at its melting point) is called the specific latent heat of fusion or melting. The term latent heat of fusion represents the same quantity. The important units of latent heat of fusion is joules per kilogram(J/kg). Other important units are cal/g or k cal/kg.

$Latent\ heat\ of\ Fusion$ = $\frac{Heat\ required}{Mass\ of\ the\ substance}$

$L$ = $\frac{Q}{m}$ J/kg

This is an endothermic change, as the process absorbs energy. For example, if a sample of ice is kept at $0^{0}C$, the amount of heat it absorbs for the whole of the ice to change into water is its latent heat of fusion. The latent heat of fusion of a given quantity of a substance can be found out using the specific latent heat of fusion. The specific latent heat of fusion is the amount of energy required to accomplish the state change of one kg of the substance without a change in the temperature of the surroundings. Latent heat of fusion of ice is 3.35 x 105 J kg-1 or 80 cal g-1.

Latent Heat of Vaporization

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It is known that when heat energy is supplied to a solid substance its temperature raises till it starts melting. Similarly when heat energy is supplied to a liquid substance its temperature raises till it starts changing into its vapour.

The quantity of heat required per unit mass of a liquid substance to completely convert it into vapour at constant temperature (i.e. at its boiling point) is called the specific latent heat of vaporisation. The term latent heat of vaporization represents the same quantity.
$Latent\ heat\ of\ vaporization$ = $\frac{Heat\ required}{Mass\ of\ the\ substance}$

$L$ = $\frac{Q}{m}$ J/kg

For water, the latent heat of fusion is 334 kJ/kg (at $0^{o}C$) and the latent heat of vaporization is 2260 kJ/kg (at $100^{o}C$)

Latent Heat of Sublimation

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The quantity of heat required per unit mass of a solid substance to completely convert it into vapour at constant temperature (i.e. at its boiling point) is called the specific latent heat of sublimation. The term latent heat of sublimation represents the same quantity.

$Latent\ heat\ of\ sublimation$ = $\frac{Heat\ required}{Mass\ of\ the\ substance}$

$L$ = $\frac{Q}{m}$ J/kg

Latent Heat vs Sensible Heat

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Heat intensity or heat level can readily be measured when it changes the temperature of a substance, with the help of a thermometer. When change of temperature registered, there must be a change in heat intensity and is called sensible heat.

Another type of heat is called latent or hidden heat. In this process heat is known to be applied, but no temperature rise is noticed. The applied heat has been used for state transformation.