Science
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Chapter 1: Matter in Our Surroundings
Class: IX
Exercise 4
Question 1
Why does a desert cooler cool better on a hot dry day?
Answer 1
The water inside the desert cooler is made to evaporate by absorbing energy from the surroundings and this makes surroundings cool. We know that evaporation depends on the humidity (it is the amount of water vapour present in air). If the humidity is less, then evaporation is more. On a hot dry day, the humidity is less and thus, the water present inside the desert cooler evaporates more, thereby cooling the surroundings more. This is the reason, on a hot dry day a desert cooler cools better.
Question 2
How does water kept in an earthen pot (matka) become cool during summers?
Answer 2
When a liquid evaporates, the liquid particles absorbs energy from the container to compensate the energy loss occurs during evaporation. This makes the remaining water in the pot cool.
The liquid inside the pot evaporates through pores in an earthen pot and this makes the water inside the pot cool. That is why; water in an earthen pot becomes cool during summers.
Question 3
Why does our palm feel cold when we put some acetone or petrol or perfume on it?
Answer 3
When we place some acetone or petrol or perfume on our palm, the liquid particles absorb energy from the surrounding or the surface of the palm and get evaporated, this makes the surroundings cool. This is why, we feel cold when we put acetone or petrol or perfume on our palm.
Question 4
Why are we able to sip hot tea or milk faster from a saucer than a cup?
Answer 4
Since the saucer has large surface area than the cup, the liquid evaporates faster and cools faster in a saucer than in a cup. This is why we are able to sip hot tea or milk faster from a saucer than a cup.
Question 5
What type of clothes should we wear in summers?
Answer 5
During the summer, we need to wear cotton clothes or light colored clothes. Since we sweat more in the summer, we need to wear clothes that absorb water well. Cotton is such kind cloth and it absorbs sweat from our body and exposes to the atmosphere, making evaporation faster. During this evaporation, particles on the surface of the liquid gain energy from our body surface, makes our body cool.
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