NCERT Class – 9 Science solution: Chapter 1 Matter in Our Surroundings-Part 6

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Question 25

You want to wear your favourite shirt to a party, but the problem is that it is still wet after a wash. What steps would you take to dry it faster?

Answer:

Conditions that can increase the rate of evaporation of water are

(a) An increase of surface area by spreading the shirt

(b) An increase in temperature by putting the shirt under the sun

(c) Increase the wind speed by spreading it under the fan.

Question 26

Comment on the following statements:

  1. Evaporation produces cooling.

  2. Rate of evaporation of an aqueous solution decreases with increase in humidity.

  3. Sponge though compressible is a solid.

Answer:

  1. Evaporation produces cooling as the particles at the surface of the liquid gain energy from the surroundings and change into vapour thereby producing a cooling effect.

  2. Air around us cannot hold more than a definite amount of water vapour at a given temperature which is known as humidity. So, if the air is already rich in water vapour, it will not take up more water therefore, rate of evaporation of water will decrease.

  3. A sponge has minute holes in which air is trapped. Also the material is not rigid. When we press it, the air is expelled out and we are able to compress it.

Question 27

Why does the temperature of a substance remain constant during its melting point or boiling point?

Answer:

  • The temperature of a substance remains constant at its melting and boiling points untill all the substance melts or boils because, the heat supplied is continuously used up in changing the state of the substance by overcoming the forces of attraction between the particles.

  • This heat energy absorbed without showing any rise in temperature is given the name latent heat of fusion/latent heat of vapourisation.

Image of Melting point and Boiling point

Image of Melting Point and Boiling Point

Image of Melting point and Boiling point

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