- The heat energy which has to be supplied to change the state of a substance is called its Latent Heat.
- Latent heat does not raise (or increase) the temperature. But Latent heat has always to be supplied to change the state of a substance. The word ‘latent’ means ‘hidden’.
- The latent heat which we supply is used up in overcoming the forces of attraction between the particles of substance during the change of state.
Latent heat is of two types:
- Latent Heat of Fusion
- Latent Heat of Vaporization
Latent Heat of Fusion (Solid to Liquid Change):
- The Latent heat of Fusion (or melting) of a solid is the quantity of heat required to convert Unit mass of Solid ( or 1 kilogram of the solid) at its melting point into liquid (without any change in temperature).
- Units of Latent Heat of Fusion is Joule/Kg or Joule / gram or Calorie/Kg or Calorie/gram.
- Similarly for Ice , the heat which is going into ice but not increasing its temperature and resulting into it’s melting is known as Latent Heat of Fusion of Ice or latent heat of melting of ice.
- The latent heat of fusion of ice is 3.34×105 Joules/kilogram (or 80 Calorie/gram).
Latent Heat of Vaporization (liquid to gas change):
- The latent heat of vaporization of a liquid is the quantity of heat required to convert Unit mass (or 1 kilogram) of the liquid at its boiling point to vapour or gas, without any change in temperature.
- Units of Latent Heat of Vaporization is Joule/Kg or Joule / gram or Calorie/Kg or Calorie/gram or Joule/mole.
- The latent heat of vaporization of water is 22.6×105 Joules/kilogram.
Note: 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 same temperature steam contains more heat, in the form of latent heat, than same temperature boiling water.
The changing of a solid directly into vapours on heating and of vapours into solid on cooling, is known as Sublimation.
- The changing of a solid directly into vapor (or gas) is called Sublimation.
- The changing of vapor (or gas) directly into solid is called Sublimation.
- The common substances which undergo sublimation are: Ammonium chloride, Iodine, Camphor, Naphthalene and Anthracene.
- When these solids are heated, their particles move so quickly that they separate completely to form vapor (or gas). And when these vapor (or gas) is cooled, these particles slow down so quickly that they become fixed and form a solid.
Effect of Change of Pressure:
The physical state of matter can also be changed by increasing the pressure or decreasing the pressure.
- Gases can be liquefied by applying pressure and lowering temperature.
- When a high pressure is applied to a gas, it gets compressed (into a small volume), and when we also lower its temperature, it gets liquefied. So, we can also say that gases can be liquefied (turned into liquids) by compression and cooling.
- Ammonia is the gas at room temperature which can be liquefied most easily by applying high pressure and lowering the temperature.
- Decreasing the pressure and raising the temperature can change the state of matter.
- Solid Carbon dioxide (dry ice) is stored under high pressure. When a slab of solid carbon dioxide is kept exposed to air, then the pressure on it is reduced to normal atmospheric pressure (1 atmosphere), its temperature rises, and its starts changing into carbon dioxide gas.