Physics

Sound

2. Latent Heat of Vaporisation (Liquid to Gas Change)

The latent heat of vaporisation can be studied by performing an experiment as follows : We take some water in a beaker and suspend a thermometer in it (see Figure 42). We heat this water by using a burner and note its temperature after every minute. As heat is given, the temperature of water rises gradually until 100°C is reached. At the temperature of 100°C, water boils and starts changing into steam (which is a gas). As more heat is given to water, more steam is formed but the thermometer reading remains at 100°C showing that there is no rise in temperature during the boiling of water. Thus,once the water has begun to boil, the temperature remains constant at 100°C until all the water has changed into steam. The heat which is going into boiling water but not increasing its temperature is the energy required to change the state of water from liquid to gas (or vapour). This is known as the latent heat of vaporisation of water. We will now give a definition of the latent heat of vaporisation of a liquid substance.
The latent heat of vaporisation of a liquid is the quantity of heat in joules required to convert 1 kilogram of the liquid (at its boiling point) to vapour or gas, without any change in temperature. It has been found by experiments that

22.5 X 10

⁵ joules of heat is required to change 1 kilogram of water (at its boiling point, 100°C) to steam at the same temperature of 100°C.So, the latent heat of vaporisation of water is $22.5 X 10$ ⁵ joules per kilogram (or

22.5 X 10

⁵ J/kg).
We have just seen that when a liquid boils (on heating), its temperature remains the same. The heat energy supplied to a boiling liquid is used up in changing the state of the liquid substance by overcoming the force of attraction between its particles. This point will become more clear from the following example of vaporisation of water.
We will now discuss why the temperature of boiling water does not rise even though heat is being given continuously. This can be explained as follows: Water is a liquid substance. The particles of water attract one another with certain forces. These forces of attraction hold the water particles together in the liquid state. The heat which we supply to water during boiling is used to overcome (or break) the forces of attraction between water particles so that they become totally free and change into a gas (steam). This latent heat does not increase the kinetic energy of water particles and hence no rise in temperature takes place during the boiling of water.If we put a little of spirit at the back of our hand and wave it around, the spirit evaporates rapidly and hand feels very cold. This is due to the fact that spirit needs latent heat of vaporisation to change from liquid to vapour state (gas state). The spirit takes the latent heat of vaporisation from our hand. So, our hand loses heat and gets cooled.
When water changes into steam, it absorbs latent heat, but when steam condenses to form water, an equal amount of latent heat is given out. It has been found that the burns caused by steam are much more severe than those caused by boiling water though both of them are at the same temperature of 100°C. This is due to the fact that steam contains more heat, in the form of latent heat, than boiling water. So,when steam falls on our skin and condenses to produce water it gives out

22.5 X 10

⁵ joules per kilogram more heat than boiling water at the same temperature. Since steam gives out more heat than boiling water,it causes more severe bums. This also explains why steam is better than boiling water for heating purposes.

Notes

Sound - Notes

1. 3. Humidity of Air