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Latent Heat





Latent heat:

  • 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 does not increase the kinetic energy of the particles of the substance, so the temperature of a substance does not rise during the change of state.

Latent heat is of two types:

  1. Latent heat of fusion
  2. Latent heat of vaporization.

Latent Heat of Fusion (solid to liquid change):

  • The heat which is going into ice but not increasing its temperature, is the energy required to change the state of ice from solid to liquid (water). This is known as the latent heat of fusion of ice (or latent heat of melting of ice).
  • The latent heat  of fusion (or melting) of a solid is the quantity of heat in joules required to convert 1 kilogram of the solid (at its melting point) to liquid, without any change in temperature.
  • The latent heat of fusion of ice is 3.34×105 joules per kilogram (or 3.34 ×105 j/kg).


Latent Heat of Vaporization (liquid to gas change):

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.
The latent heat of vaporization of water is 22.5×105 joules per kilogram (or 22.5×105 j/kg).
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 steam contains more heat, in the form of latent heat, than boiling water.

Sublimation:

The changing of a solid directly into vapours on heating and of vapours into solid on cooling, is known as sublimation.
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  1. The changing of a solid directly into vapor (or gas) is called sublimation.
  2. 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.


Test your Knowlegde

Fill in the blanks
1)the movement of particles is the __________________ in solids, more in liquids and the maximum in gases.
2) The spreading out and mixing of a substance with another substance due to the motion of its particles is called ________________
3) Diffusion is the __________________ of matter which is based on the motion of its particles.
4) Diffsion  is fastest in ______ and slowest in _____
5) The rate of diffusion increases on increasing the __________________ of the diffusing substance.
6) The rate of diffusion of a gas depends on its __________________.
7) The ________ and oxygen gas present in air  diffuse into water, and dissolve in it.
8) The aquatic plants use the dissolved carbon dioxide for preparing __________________ by photosynthesis and aquatic animals use the dissolved oxygen of water for  breathing.
9) The process in which a solid substance converts into a liquid on heating,is called  ___________
10) The temperature at which a solid substance melts and changes into a liq uid at atmospheric  pressure,is called  __________________.
11)Higher the melting point of a solid substance, greater will be the __________________ of attraction between its particles.
12)  At the melting point, the particles of a solid have sufficient kinetic __________________ to overcome the strong forces of attraction holding them in fixed positions and break to form small groups of particles and the solid melts to form aliquid.
13) The process in which a liquid substance changes  into a gas rapidly on heating, is called _________
14) The process of changing a __________________ (or vapour) to a liquid by cooling is called condensation.
15) The process of changing a __________________ into a solid by cooling is called freezing.
16) Latent heat does not raise (or increase) the __________________.
17) The __________________   of a solid is the quantity of heat  in joules required to convert 1 kilogram of the solid (at its melting point) to liquid, without any change in temperature.
18) The latent heat of fusion  of ice is _____ joules per kilogram
19) The __________________  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.
20) The latent heat of vaporization of water is __________ joules per kilogram
21) The changing of a solid directly into vapours on heating and of vapours into solid on cooling, is known as __________________.
22) The three common substances which undergo sublimation are: ______,______ and _______
23) When these solids are heated, their particles move so quickly that they separate completely to form __________________ (or gas).
24) And when these vapor (or __________________) is cooled, these particles slow down so quickly that they become fixed and form a solid.






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