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CHANGES IN THE STATE OF MATTER

Change in the state of matter is accomplished when the temperature of solid is increased.

The particles acquire sufficient energy to disrupt the ordered arrangement of the molecules and convert to the liquid state. As the temperature is further increased the distance between the molecules is enormously increased and resulted in the free movement of the molecules and pass into the gaseous state.

Various physical phenomenons are occurred during the change in the state of matter due to change in entropy and the enthalpy. They are as follows:

Melting: When a solid turns to liquid due to increase in temperature.

Freezing: When a liquid turns to solid due to decrease in temperature.

Vaporization: When liquid turns to vapours due to increase in temperature.

Condensation: when vapours turns to liquid due to decrease in temperature.

Sublimation: when the solid turns to gas without passing through the intermediate liquid phase at the temperature and pressure below the substance’s triple point.

Deposition: The reverse of sublimation is called the deposition i.e., conversion of gas to solid.

While conversion from solid to liquid and from liquid to gas state, the enthalpy (heat content) of the material is also increased. Thus enthalpy of liquid is greater than the solid and the enthalpy of gas is greater than that of liquid. Similarly, entropy (degree of molecular randomness) also increase with the conversion from solid to liquid and then to gas.

LATENT HEAT

When change in the state of mater occurs, temperature does not rise, it remains constant but heat get absorbed or liberated. Thus, the heat which results in the change of matter, 9r without change in the temperature is called the latent heat.

Latent heat of Fusion:

This heat is latent because it is not associated with the change in temperature. Latent heat of fusion is associated with the solid-liquid transition without change in the temperature.

For example, ice (at 0°C or 32°F) converts to water with the latent heat of fusion.

Ice (at 0°C) converts to water with the latent heat of fusion. The heat of fusion for water at 0°C is approximately 80 calories per gram of ice.

Latent heat of vaporization:

Latent heat of vaporization is associated with liquid-vapour transition. The amount of heat required to convert unit mass of a liquid into the vapour without a change in temperature.

The latent heat of vaporization of water at 100°C is approximately 533 calories per gram of liquid.

VAPOUR PRESSURE

There is a tendency of liquid to evaporate constantly. It takes place at the surface of the o liquid. The liquid molecules escape into the space above the surface when kinetic energy of liquid molecules overcomes the intermolecular forces of attraction in the liquid state.

This process is called ‘evaporation’. If evaporation is carried out in a closed container system then the vapors of liquid remains in contact with surface of the liquid and accomplish continuous random motion. During this motion, molecules collide with each other and also with walls of the container. Due to this activity, molecules losses their energy and returns back to liquid state. This process is called as ‘condensation’.

The phenomenon of evaporation and condensation is a constant process and after some time equilibrium is established at constant temperature between evaporation and the condensation. At equilibrium number of molecules in vapor state remains constant at constant temperature.

Thus, the pressure exerted by vapors of liquid on the surface of liquid when equilibrium is established between liquid and its vapor is called vapor pressure of the liquids. The vapor pressure of liquid depends on nature of the liquid and the temperature. The vapor pressure of liquid decreases with the increase in the intermolecular forces of attraction but it increases with the increase in the temperature. Vapor pressure can be measured in different units’ viz., (kPa (kilo pascal), atm, bar, mm Hg, torr).