Page 174 - ICSE Chemistry 8

Pure water, being transparent, allows light to pass through and helps aqua c plants to carry out
photosynthesis. This feature also helps aqua c animals to see under water.
• Boiling point: Under normal pressure, pure water boils at 100°C and changes into steam. The
temperature remains constant ll all the water has changed into steam. This constant temperature
is called boiling point. The boiling point of water varies with pressure in the following manner.

 With decrease in pressure, the boiling point of water decreases. This is because when less
pressure is ac ng on the surface of a liquid, the liquid can escape as water vapour more easily.
That is why, water boils at lower temperature in the hills, where the atmospheric pressure is
lower than in the plains. Hence, it takes a longer me
to cook food in hilly regions. Thirst for Knowledge

 With increase in pressure, the boiling point of water Water has very high specifi c
increases. This is because when more pressure is heat which means that it can
ac ng on the surface of a liquid, the liquid cannot absorb or lose much heat before
escape as water vapour easily. That is why, in a its temperature changes. This is
pressure cooker the increased pressure raises the important in main taining body
boiling point of water and food cooks faster. heat in mammals.

Activity 8.2

Aim: To determine the boiling point of water
Materials required: Some dis lled water, a beaker, a tripod stand, a 100°C
100
burner, a thermometer

boi
Procedure boiling water
• Pour some dis lled water in a beaker.

• Set up the apparatus as shown alongside. Ensure that the bulb of
the thermometer dips in the water and does not touch the sides of
Determining the boiling point of water
the beaker.
• Note the temperature of water when it starts boiling.
Observa on: Water starts boiling at 100° C. The temperature then becomes constant.

Conclusion: This shows that boiling point of pure water is 100° C.

• Freezing point: Under normal pressure pure water
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freezes at 0°C and changes into ice. The temperature
remains constant ll all the water has changed into If there are impuri es in water, then its
ice. This constant temperature is called freezing freezing point decreases and its boiling
point. With increase in pressure, freezing point of point increases.
water decreases.

Let’s perform Ac vity 8.3 to observe the eff ect of changing pressure on the freezing point of water.

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