Heat and change of physical 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).