Boiling and condensation

Show/Hide Sub-topics (Thermal Physics | O Level)

Boiling is the change of state from a liquid to a gas. Boiling of a pure substance occurs at a particular constant temperature called boiling point.

The change of state from a gas to a liquid is called condensation. A pure substance condenses at a temperature equal to its boiling point.

From the graph above,

• $100^{\circ}\text{C}$ is the boiling point of the liquid
• Temperature remains constant at $100^{\circ}\text{C}$ as the liquid is boiling
• During the boiling process, liquid and gas exist in equilibrium.

Step by step of what happens during boiling:

1. Heat energy is absorbed by the particles
2. The kinetic energy of the particles increases
3. At boiling point, the particles have enough energy to overcome the attractive forces between the particles and move away from one another.
4. The particles escape from the liquid to form gas

Cause for constant temperature during boiling: The absorbed heat energy is used to overcome the attractive forces between the particles and not the kinetic energy.

Factors That Affect Boiling Point

Adding impurities to a liquid affect its boiling point. E.g. By adding a 100 g of salt to 1000 g of water increases its boiling point by $1^{\circ}\text{C}$

When the liquid vapourises, volume of the liquid expands. High pressure applied to liquid will oppose its expansion into gas and thus, opposes boiling. Therefore, the boiling point of the liquids increases with increasing pressure. A reduction in pressure lowers the boiling point.

• In mountains, water boils at lower temperatures as the atmospheric pressure decreases with the elevation.
• Evaporated milk is produced by boiling the milk under reduced pressure. Boiling under reduced pressure saves energy required to boil and the lowered temperature preserves the vitamins in the milk better.
• In pressure cookers, when the cooking pressure increases and the boiling point of water inside the cooker increases. Therefore, higher temperature can be achieved and the food can be cooked more quickly.

Note: The boiling point of pure water is 100 °C at standard atmospheric pressure.

5 thoughts on “Boiling and condensation”

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