# Latent Heat of Vaporization and Latent Heat of Steam by Mixture Method | Physics Grade XI

### Latent Heat of Vaporization

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#### Viva Voice Questions # Latent Heat of Vaporization and Latent Heat of Steam by Mixture Method

Latent heat of Vaporization
The latent heat of vaporization of liquid is defined as the amount of heat required to change the unit mass of liquid at boiling point into vapor at the same temperature. Latent heat of steam is defined as the amount of heat required to change a unit mass of waster from 100 degree Celsius to steam at the same temperature.

The latent heat of Vaporization for steam is 2.26 * 106 J kg-1 or 540 Cal g-1. Latent heat of steam by mixture method
Let us consider a clean and dry calorimeter and fill it about one third with water. Suppose θ10C is the initial temperature of the calorimeter and its content. On the next side, steam is produced in the boiler and that steam is supplied to the calorimeter as shown in the figure. After some time, the temperature of the calorimeter and its content is increased. Suppose the final temperature of mixture is θ20C. Here steam loses heat and the calorimeter and water gain heat. So, the amount of heat gain by calorimeter and water from θ10C to θ20C
= m1s (θ2 – θ1) + m2 1 (θ2 – θ1)
= (m1 s + m2) (θ2 – θ1) …... (i)
Where m1 is the mass of calorimeter with the stirrer, s is the mass-specific heat of calorimeter with the stirrer, and m2 is the mass of water.

i. When steam changes from 1000C to water 1000C i.e. suppose θ30C. Then the amount of heat lost by steam = mL ….... (ii)
Where m is the mass of steam, L is the latent heat of steam
ii. When water (just formed from steam) changes from θ30C to water at θ20C.

Amount of heat loss by water from θ30C to θ20C = m. sw3 – θ2)
.: The total amount of heat lost by the steam = mL + m sw3 – θ2)

From the principle of calorimetry, we have
Heat lost by steam = heat gained by water
mL + m (θ3 – θ2) sw = (m1 s + m2) (θ2 – θ1)
.: L = [(m1 s + m2) (θ2 – θ1)/ m] - (θ3 – θ2) sw
From this relation, we can determine the latent heat of the steam

When a solid change into liquid or liquid changes into solid, the change of state takes place. The change of state from liquid to solid is known as freezing and the change of state from solid to liquid is called melting.

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