Change of State

When heat is added or removed from a substance, its physical state can change. Substances can exist in different forms called physical states: solid, liquid, or gas.

1. Solid to liquid: As the temperature of solids increases, the kinetic energy of the particles increases, which overcomes the forces of attraction between the particles; thereby, the solid is converted to a liquid.

• Melting: The change of the solid state of a substance into a liquid is called melting.
• Melting point: The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point of ice is 0 °C.

2. Liquid to gas: When a liquid like water is heated, the kinetic energy of its particles increases as high as in a gas, causing the liquid to change to a gas.

• Boiling: Changing a liquid substance into gas when heating is called boiling.
• Boiling point: The temperature at which a liquid boils and changes rapidly into a gas at the atmospheric pressure is called its boiling point. The boiling point of water is 100 °C.

3. Gas to liquid: When a gas like steam (or water vapour) is cooled, the kinetic energy of its particles is lowered, causing them to move slowly and bringing them closer, forming a liquid.

• Condensation: The process in which a gas turns into a liquid at a specific temperature upon cooling is called condensation or liquefaction.

4. Liquid to solid: When a liquid is cooled down by lowering its temperature, its particles lose kinetic energy and become stationary, causing the liquid to turn into a solid.

• Freezing: The change of a liquid substance into a solid by lowering its temperature is called freezing.
• Freezing point: The temperature at which the state of a substance changes from a liquid to a solid is called the freezing point of that substance.

5. Fusion: The process of melting, that is, the change of a solid state into a liquid state, is also known as fusion.

6. Latent heat: The heat energy required to change a substance's state without causing any change in the substance's temperature is called latent heat. Since the heat energy is hidden in the bulk of the matter, it is called latent heat.

• Latent heat of fusion: The heat energy required to convert 1 kilogram of a solid into a liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• Latent heat of vaporisation: The heat energy required to convert 1 kilogram of liquid into gas at atmospheric pressure, at its boiling point, is known as the latent heat of vaporisation. Water vapour at 373 K has more energy than water at the same temperature because particles in steam have absorbed extra energy in the form of latent heat of vaporisation.

7. Sublimation: The change of state of a substance directly from a solid to gas or gas to solid, without changing into the liquid state, is called sublimation.

• Pressure does not affect solids or liquids because both states of matter are non-compressible. But if pressure is increased on a solid, it breaks.
• On the other hand, applying pressure at a reduced temperature can liquefy gases. For example, during parties or stage shows, you must have noticed smoke that spreads all around the stage.
• It is nothing but dry ice (solid carbon dioxide). Solid carbon dioxide is stored under high pressure and liquefies instantly as soon as the pressure is reduced to 1 atmospheric pressure.

A specific amount of heat must be gained or lost for a substance to change its state.

• For example, to melt ice, you need to add heat until the ice becomes water.
• To freeze water, you need to remove heat until the water turns into ice.

The temperature at which these changes occur is important.

For example:

• Water melts at 0°C (ice to water).
• Water boils at 100°C (water to steam).

If a substance gains heat, it becomes hotter and changes its state (solid to liquid or liquid to gas). If a substance loses heat, it becomes colder and changes its state (from gas to liquid or liquid to solid).

The process of change from one state to another at a constant temperature is called the change of phase.

The change from solid to liquid phase is known as melting, while the reverse change from liquid to solid is called freezing. 

The direct change from solid to vapour is called sublimation.

The change from vapour to solid is called solidification.