Page 18 - Understanding NCERT Science 09

• Start heating the beaker on a low flame. state by overcoming the forces of attraction
• Note the temperature when the ice between the particles. As this heat energy is
starts melting. absorbed by ice without showing any rise in
• Note the temperature when all the ice temperature, it is considered that it gets
has converted into water. hidden into the contents of the beaker and is
• Record your observations for this known as the latent heat. The word latent
conversion of solid to liquid state.
means hidden. The amount of heat energy
• Now, put a glass rod in the beaker and that is required to change 1 kg of a solid into
heat while stirring till the water starts liquid at atmospheric pressure at its melting
boiling. point is known as the latent heat of fusion.
• Keep a careful eye on the thermometer So, particles in water at 0 C (273 K) have more
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reading till most of the water has energy as compared to particles in ice at the
vaporised.
• Record your observations for the same temperature.
conversion of water in the liquid state When we supply heat energy to water,
to the gaseous state. particles start moving even faster. At a certain
temperature, a point is reached when the
On increasing the temperature of solids, particles have enough energy to break free
the kinetic energy of the particles increases. from the forces of attraction of each other. At
Due to the increase in kinetic energy, the this temperature the liquid starts changing
particles start vibrating with greater speed. into gas. The temperature at which a liquid
The energy supplied by heat overcomes the starts boiling at the atmospheric pressure is
forces of attraction between the particles. The known as its boiling point. Boiling is a bulk
particles leave their fixed positions and start phenomenon. Particles from the bulk of the
moving more freely. A stage is reached when liquid gain enough energy to change into the
the solid melts and is converted to a liquid. vapour state.
The minimum temperature at which a solid For water this temperature is 373 K
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melts to become a liquid at the atmospheric (100 C = 273 + 100 = 373 K).
pressure is called its melting point. Can you define the latent heat of
vaporisation? Do it in the same way as we
The melting point of a solid is an indication have defined the latent heat of fusion.
of the strength of the force of attraction Particles in steam, that is, water vapour at
0
between its particles. 373 K (100 C) have more energy than water
at the same temperature. This is because
The melting point of ice is 273.15 K*. The particles in steam have absorbed extra energy
process of melting, that is, change of solid in the form of latent heat of vaporisation.
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 So, we infer that the state of matter can
of the system does not change after the be changed into another state by changing
melting point is reached, till all the ice melts. the temperature.
This happens even though we continue to We have learnt that substances around
heat the beaker, that is, we continue to supply us change state from solid to liquid and from
heat. This heat gets used up in changing the liquid to gas on application of heat. But there

*Note: Kelvin is the SI unit of temperature, 0 C =273.15 K. For convenience, we take 0 C = 273 K after
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rounding off the decimal. To change a temperature on the Kelvin scale to the Celsius scale you
have to subtract 273 from the given temperature, and to convert a temperature on the Celsius
scale to the Kelvin scale you have to add 273 to the given temperature.
MATTER IN OUR SURROUNDINGS 7

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