NCERT SOLUTION CLASS 9 SCIENCE PAGE-10

NCERT SOLUTION OF CHAPTER 1 OF CBSE BOARD CLASS 9 ( MATTER IN OUR SURROUNDINGS)

PAGE NO 10

 QUESTION NO 1

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

 ANSWER NO 1

    PRINCIPLE:  

     When a liquid evaporates, the particle of the liquid absorb energy from the surroundings to compensate the loss of energy during evaporation. This makes the surroundings cool.

                              In desert cooler, the water from inside the cooler evaporate. This lead to absorption of energy from the surroundings, hence it cools the surroundings. In hot dry day the amount of water vapour present in air less, thus water present inside the cooler evaporates more, hence cools the surroundings.

    QUESTION NO 2 

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

     ANSWER NO 2

  There are some pores in an earthen pot through which the liquid inside the pot evaporates, hence the evaporated particle of the liquid absorb energy from the pot to compensate the loss of energy during evaporation. This evaporation makes the water inside the pot cool.

     QUESTION NO 3

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

      ANSWER NO 3

     When we put some acetone or perfume or petrol on our palm, it evaporates. During evaporation, particles of liquid absorb energy from the surrounding and the surface of the palm to compensate the loss of energy, making the palm cool.

      QUESTION NO4 

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

      ANSWER NO 4 

     In a saucer the surface area of tea is greater than the cup.

 Hence it evaporates faster in a saucer and cools the hot tea faster in compare to cup. Because of this reason we are able to sip hot tea or milk faster from a saucer than a cup.

     QUESTION NO 5  

     What type of clothes should we wear in summer? 

      ANSWER NO 5

     We should wear cotton clothes in summer. We sweat more in summer and cotton clothes are good absorber of water. It absorb sweat from the body and expose the liquid to the atmosphere and evaporation takes place. During evaporation, particles on the surface of the liquid gain energy from our body surface, making the body cool.