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Change of State of Matter - Effect of Change of Temperature

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1. Activity :

• Take about 150 g of ice in a beaker and suspend a laboratory thermometer so that its bulb is in contact with the ice, as in Fig. 1.6.

(a) Conversion of ice to water, (b) conversion of water to water vapour

• Start heating the beaker on a low flame.
• Note the temperature when the ice starts melting.
• Note the temperature when all the ice has converted into water.
• Record your observations for this conversion of solid to liquid state.
• Now, put a glass rod in the beaker and heat while stirring till the water starts boiling.
• Keep a careful eye on the thermometer reading till most of the water has vaporised.
• Record your observations for the conversion of water in the liquid state to the gaseous state.

On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. 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 273.16 K*. The process of melting, that is, change of solid state into liquid state is also known as fusion. When a solid melts, its temperature remains the same, so where does the heat energy go? You must have observed, during the experiment of melting, that the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat. The word latent means hidden. The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. So, particles in water at 00 C (273 K) have more energy as compared to particles in ice at the same temperature. 

2. Activity : 

• Take some camphor or ammonium chloride. Crush it and put it in a china dish.
• Put an inverted funnel over the china dish.
• Put a cotton plug on the stem of the funnel, as shown in Fig. 1.7.

Fig. Sublimation of ammonium chloride

• Now, heat slowly and observe.
• What do you infer from the above activity?

A change of state directly from solid to gas without changing into liquid state (or vice versa) is called sublimation.

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