Matter In Our Surrounding

• Latent heat is the heat energy which is supplied to change the state of a substance.
• Latent heat does not cause a rise in temperature of the substance.
• The latent heat which we supply is used up in overcoming the forces of attraction between the particles of a substance during the change of state.
• The latent heat does not increase the kinetic energy of the particles of the substance.
• Since, there is no increase in the kinetic energy of the particles; the temperature of a substance does not rise during the change of state.
• Latent heat is of two types:
  • Latent heat of fusion
  • Latent heat of vaporisation

Latent Heat of Fusion (Solid to Liquid Change):

• The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• For instance, the heat energy absorbed by ice to change the state of ice from solid to liquid, without showing any rise in temperature depicts latent heat of fusion.
• Here the heat is considered to be hidden into the contents of the beaker and is known as the latent heat.
• When a solid melts, it absorbs heat to form liquid. The reverse of that is also true.
• That is, when a liquid freezes to form a solid, an equal amount of heat is given out.

Latent Heat of Vaporisation (Liquid to Gas Change):

• The latent heat of vaporisation of a liquid is the quantity of heat in joules required to convert kilogram of the liquid (at its boiling point) to vapour or gas, without any change in temperature.

SUBLIMATION

• A change of state directly from solid to gas without changing into liquid state is called sublimation.
• The common substances which undergo sublimation are Ammonium chloride, Iodine, Camphor, Naphthalene and Anthracene.
• Solid CO₂ gets converted directly to gaseous state on decrease of pressure to  1 atmosphere without coming into liquid state.
• This is the reason that solid carbon dioxide is also known as dry ice.

Effect of Change of Pressure:

• The physical state of matter can also be changed by changing the pressure.
• Gases can be liquefied by applying pressure and lowering temperature.
• Ammonia gas can be liquefied by applying high pressure and lowering the temperature.
• Solid carbon dioxide (dry ice) is stored under high pressure.