Change of State

Change of State

Any material can remain in any of its three states (solid, liquid and gas). To change the substance from one state to other state is called change of state. For this either substance is heated or heat is extracted from the substance. Change of state takes place at a fixed temp.

Fusion

The process by which a substance is changed from solid state to liquid state is called fusion. Fusion takes place at a fixed temperature called melting point (M.P.)

Freezing

The process by which a substance is changed from liquid state to solid state is called freezing. Freezing takes at a fixed temperature called freezing point. (F.P.)
For a substance M.P. = F.P.
» M.P. of a substance changes with the change in pressure. Melting point of substances which contracts in the process of fusion (ais ice) decreases with the increase in pressure. Melting point of substances which expands in the process of fusion (as wax) increases with the increase in pressure.
» With the addition of impurity (as salt in ice), melting point of a substance decreases.

Vapourisation

The process by which a substance is changed from liquid state to vapour state is called vapourisation.
Vapourisation takes place by two methods :
1. Evaporation
2. Boiling or Ebullition

1. Evaporation :
The process of vapourisation which takes place only from the exposed surface of liquid and that at all temperatures is called evaporation.
» Evaporation causes cooling. This is why water in a earthed pot gets cooled in summer

2. Boiling : The process of vapourisation which takes place at a fixed temperature from whole part of liquid is called boiling. The temperature at which boiling takes place is called boiling point.

Condensation

process by which a substance is changed from vapour state to liquid state is called condensation.

» Boiling point of a liquid increases with the increase in pressure.
» Boiling point of a liquid increases with the addition of impurity.

Latent heat or heat of transformation

The amount of heat required to change the state of unit mass of substance at constant temperature is called latent heat.
If Q heat is required to change the state of a substance of mass m at constant temperature and L is the latent heat, then Q = mL.
» S.I. unit of latent heat is Joule/kilogram.


Any material has two types of latent heat
1. Latent heat of fusion
2. Heat of vapourisation

1. Latent heat of fusion
It is the amount of heat energy required to convert unit mass a substance from solid state to liquid state at its melting point. It is also the amount of heat released by unit mass of liquid when changed into Solid at its freezing point.

2. Heat of vapourisation
It is the amount of heat required to change unit mass of a substance from liquid state to vapour state at its boiling point. It is also of the amount of heat released when unit mass of a vapour is changed into liquid.

Sublimation

Sublimation is the process of conversion of a solid directly into vapour.
» Sublimation takes place when boiling point is less than melting point.
» Sublimation is shown by camphor or ice in vacuum.

Hoar Frost

Hoar frost is just the reverse process of sublimation i.e. it is the process of direct conversion of vapour into solid.
» Steam produces more severe burn than water at same temperature because internal energy of steam is more than that of water at same temperature.

Relative Humidity

Relative humidity is defined as the ratio of amount of water vapour present in a given volume of atmosphere to the amount of water vapour required to saturate the same volume at same temperature.
The ratio is multiplied by 100 to express the relative humidity in percentage.
» Relative humidity is measured by Hygrometer.
» Relative humidity increases with the increase of temperature.