# Latent Heat | Definition, Formula, Units – Thermometry and Calorimetry

Latent Heat Definition: (Change of State)
The heat energy absorbed or released at constant temperature per unit mass for change of state is called latent heat.

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## Latent Heat | Definition, Formula, Units – Thermometry and Calorimetry

Latent Heat Formula Physics:
Heat energy absorbed or released during change of state is given by

Q = mL

where,
m = mass of the substance and
L = latent heat

Latent Heat Unit:
Its unit is cal/g or J/kg

Latent Heat Dimensional Formula:
Dimensional formula is [L2 T-2].

Latent Heat of Vaporization of Water:
For water at its normal boiling point or condensation temperature (100°C), the latent heat of vaporization is

L = 540 cal/g = 40.8 kJ/ mol
= 2260 kJ/kg

Latent Heat of Fusion of Water:
For water at its normal freezing temperature or melting point (0°C), the latent heat of fusion is

L = 80 cal/ g = 60 kJ/mol
= 336 kJ/kg

It is more painful to get burnt by steam rather than by boiling water at 100°C. Steam converted to water at 100°C, then it gives out 536 cal of heat, so, it is clear that steam at 100°C has more heat than water at 100°C (i.e. boiling of water).

After snow falls, the temperature of the atmosphere becomes very low. This is because the snow absorbs the heat from the atmosphere to melt down. So, in the mountains, when snow falls, one does not feel too cold but when ice melts, he feels too cold.

There is more shivering effect of icecream on teeth as compared to that of water (obtained from ice). This is because when icecream melts down, it absorbs large amount of heat from teeth.

Thermometry and Calorimetry:
The thermometer is a device used to check the temperature of an object. This branch of measurement of the temperature of a substance is called thermometry. It is measured in degrees or Fahrenheit, usually.

Calorimetry also means the measurement of heat but in joules. It states the amount of heat lost by the body is the amount of heat gained by its surrounding.