Lesson 1, Topic 2
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Change of State

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Change of state is brought about by a change in temperature (i.e. heating and cooling). Substances change from solid to liquid and to gas.

When substances are heated, the particles acquire more kinetic energy. But when cooled, they become less energetic.

Change of State and Kinetic theory

1. Melting: When a solid is heated, it acquires more kinetic energy and melts. If there is impurity present in the solid, it will lower the melting point of the solid.

2. Boiling: When a liquid is heated, the saturated vapour pressure of the liquid increases, until it gets to a temperature which is equal to atmospheric pressure. At this point, bubbles of vapour form on the surface of the liquid. The liquid is said to boil. The temperature at which the liquid boils is known as the boiling point of the liquid.

Impurity in a liquid increases the boiling point of the liquid.

3. Evaporation: Evaporation occurs at all temperatures but the rate of evaporation increases with an increase in temperatures. This is because the average kinetic energy of a liquid is greater at a higher temperature. So there are more particles with sufficient energy to evaporate.

Differences between Boiling and Evaporation

4. Vapour Pressure: When a glass cover is placed on a beaker half-filled with a liquid and kept at a constant temperature T1 as shown in the figure below.

Since the beaker is covered, there is no chance for the vapour particles to escape. The vapour particles accumulate above the liquid. The particles collide with one another and with the walls of the container exerting a pressure known as vapour pressure. Some of the particles also settle at the cover of the container i.e condensation.

An equilibrium is established when the liquid particles evaporate and vapour particles condense. At this point, the vapour pressure stops rising and remain constant. This is known as saturated vapour pressure.

5. Condensation: This is a process whereby vapour loses some of its kinetic energy to the cooler body and changes into a liquid state. Condensation is the reverse process of vaporization.

6. Freezing: When you put a liquid into a freezer, the temperature will continue dropping until it reaches the freezing point. At this point, it changes completely to solid. The freezing point of water is 00C. In freezing, energy in the water is absorbed by its surroundings.

7. Latent Heat of Fusion: This is the energy absorbed by the solid as it turns into liquid to weaken the cohesive forces during melting. It is not detectable by a thermometer. This is because during melting no temperature rise is observed although heat energy has been supplied.

8. Latent Heat of Vaporisation: This is the energy absorbed by a liquid as it turns into gas to overcome the cohesive force during boiling. It is not detectable by a thermometer because the energy is not used to increase the temperature.

Heating Curve

A heating curve describes the variation of temperature with time as heat is uniformly added to a solid. When a solid is heated, there is a gradual increase in the average kinetic energy of the particles, so the temperature also increases.

Fig.—– The Heating Curve of Water

At t1 the heat absorbed increases the potential energy of the particles. The temperature Ti remains constant. This is represented by the horizontal line AB. The temperature at which the solid and liquid co-exist is called melting point.

Solid  + Heat  â‡Œ Liquid

As heating continues, the temperature rises from t2 to t3, at time t3, the heat absorbed increases the potential energy of the particles. The temperature t2 does not change, this is represented by the horizontal line CD. The temperature at which liquid and gas co-exist is called the boiling point.

Liquid   +   Heat     â‡Œ    Gas

At t4 to t5, the liquid changes completely to gas.

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