Chapter 1 : Matter in Our Surroundings : Class - 9 : Question and Answer (Part -2)

 

NCERT BOOK QUESTIONS AND EXERCISES

Chapter 1 : Matter in Our Surroundings

Part – 2

Question 9 . Convert to following temperature to Celsius scale :

(a)           300 K                          (b)    573 K

Answer : (a) We know that :

Temperature on Celsius  scale = temperature on Kelvin  scale – 273

                                        = 300 – 273

                                        = 27^oC

(b)          We know that :

Temperature on Celsius  scale = temperature on Kelvin  scale – 273

                                        = 573 – 273

                                        = 300^oC

 

Question 10 : What is the physical state of water at :

(a)          250^oC                          (b) 100^oC

 

Answer : (a) The boiling point of water is 100^oC. So, the physical state of water at a temperature of 250^oC (which is much above its boiling point) will be gaseous state.

(b)          The physical state of water at its boiling point temperature of 100^oC will be both ‘liquid state’ as well as ‘gaseous state’. This is because at its boiling point the liquid state of water starts changing in to its gaseous state(steam).

 

Question 11 : For any substance, why does the temperature remain constant during the change of state ?

Answer : The heat energy supplied to a substance during the change of state (at its melting point or boiling point ) is all used up in overcoming the force of attraction between its particles without increasing its kinetic energy. Since the heat supplied during the change of state does not increase the kinetic energy of the substance, therefore no rise in temperature takes place. The temperature remains constant.

 

Question 12 : Suggest a method to liquefy atmospheric gases ?

 

Answer : Atmospheric gases can be liquefied by applying pressure and lowering temperature. When enough pressure is applied, the gases are highly compressed in to a small volume. The particles of gases get so close together that they start attracting one another sufficiently to form a liquid. When a gas is compressed too much  by applying high pressure, a lot of heat is produced. So while applying pressure to liquefy gases, it is necessary to lower their temperature to take away the heat produced during compression. 

 

 

Question 13. Why does a desert cooler cool better on a hot dry day?

Answer – The cooling in a desert room cooler is caused by the evaporation of water. A desert cooler cools better on a hot and dry day because the higher temperature on a hot day increases the rate of evaporation of water, and the dryness of air (low humidity of air) also increases the rate of evaporation of water. And due to increased rate of evaporation of water, a desert room cooler cools better on a hot and dry day.

 

Question 14 – How does the water kept in an earthen pot (matka) become cool during summer?

Answer – The earthen pot has a large number of extremely small pores in its walls. Some of the water kept in the earthen pot continuously keeps seeping through these pores to the outside of the pot. This water evaporates continuously by taking the latent heat of vaporisation from the earthen pot and the remaining water. In this way the earthen pot and remaining water lose heat and get cooled.

 

Question 15 – Why does our palm feel cold when we put some acetone or perfume on it ?

Answer – Acetone, petrol and perfume are volatile liquids. When we put some acetone, petrol or perfume on our palm, the acetone, petrol or perfume evaporate rapidly and our palm feels cold. This is due to the fact that to change from the liquid to the vapour state, acetone, petrol or perfume require latent heat of vaporisation. The acetone, petrol or perfume take this latent heat of vaporisation from our palm. The palm loses heat and feels cold.

 

Question 16 – Why are we able to sip hot tea or milk faster from a saucer than a cup ?

Answer – If the hot tea or milk is taken in a cup then due to the narrow shape of the cup, the surface area of hot area or milk in the cup is comparatively small. Because of small surface area the evaporation of hot tea or milk taken in the cup is slow, cooling caused by evaporation is less, and hence the hot tea or milk remains appreciably hot for a much longer time, making it difficult to sip. On the other hand, saucer has a large surface area. Due to the large surface  area of hot tea or milk taken in the saucer, the evaporation of hot tea or milk from the saucer is faster. The faster evaporation cools the hot tea, or milk much more quickly making it convenient to sip.

 

Question 17 – What type of clothes should we wear in summer ?

Answer – We should wear cotton clothes in hot summer days to keep us cool and comfortable. This is due to the following reason : During hot summer days, we perspire more(give out more sweat through the pores of the skin). Sweat (pasina) is mainly water. The cotton clothes are good absorber of sweat. So, cotton clothes absorb the sweat produced on our skin quickly and expose it to the atmosphere for evaporation. The evaporation of sweat from cotton clothes takes the latent heat of vaporisation from our skin. In this way, our skin loses heat and makes us feel cool and comfortable.

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