Q1. Explain why moisture gathers on the outside of a glass tumbler containing cold water?
Solution: It is because the water vapors in the air get cooled and appear as droplets of water coming in contact with the cold surface of the glass tumbler. 
Q2. Why does a thermometer kept in boiling water show no temperature change even when the water is continuously heated?
Solution: A liquid boils at a particular temperature called the 'boiling point' of the liquid.
Once the liquid starts boiling, the thermometer reading remains constant; it's already reaching the boiling point, and because of it, there is no further increase in temperature.
The quantity of heat supplied is utilized as latent heat in converting the liquid at boiling point into vapor at the same temperature.
That is why a thermometer kept in boiling water shows no temperature change, even when the water is continuously heated.
Q3. Explain why when a highly pumped-up bicycle tire is left in the hot sun, it may burst.
Solution: All gases expand on heating. When a highly pumped-up bicycle tire is left in the hot sun, there occurs a considerable increase in the air volume.
As sufficient space for the expansion of the air is not available (because the bicycle tire is already highly pumped up), it may result in the bursting of the tire. 
Q4. Why does the ice not readily melt when salt is sprinkled over it?
Solution: When salt is sprinkled over ice, some of it dissolves.
As the dissolution of the salt is accompanied by absorption of heat, the system's temperature will fall below 0°C. Hence ice does not melt readily. 
Q5. Why are cloudy days cooler but cloudy nights warmer than the clear ones?
Solution: Because clouds do not allow the sun rays to fall on earth.
Moreover, clouds can absorb more heat radiations as compared to dry air. Both these factors prevent becoming too the earth from much heated.
Hence cloudy days are comparatively cooler. 
Q6. Why will a white roof keep a house cooler in summer than will a blacktop?
Solution: White roofs will reflect more and absorb fewer heat rays, whereas black roofs will absorb more and reflect fewer heat rays.
Hence a white roof will keep the house cooler in summer.
Q7. Why does a perspiring man feel relief when air flows by his side?
Solution: The flow of air increases the rate of evaporation of sweat from the body.
During evaporation, somebody's heat is taken away, thus giving a feeling of coolness in the body and providing relief to the perspiring person. 
Q8. Why is the water in an open pond excellent, even on a hot day?
Solution: This is because cooling is caused by evaporation.
As the water evaporates from the surface of a tank or pond, a good deal of heat is taken away.
This results in lowering the temperature of the remaining water.
Q9. Why is mercury used in Thermometers?
Solution: Mercury is used as a thermometric liquid on account of the following reasons :
i) It does not wet glass.
ii) It has a low vapor pressure at ordinary temperatures.
iii) It is opaque and can be easily seen in the thermometric tube.
iv) It is a good conductor of heat and, therefore, responds more rapidly to changes in temperature.
v) It is available in a pure state.
vi) Its specific heat is low. Hence it requires only a tiny amount of heat to raise its temperature.
vii) Its freezing point is 39°C and boiling point 357°C. Therefore, it can be used to measure low as well as high temperatures.
viii) Its expansion is uniform. Hence it is more accurate to use.
Q10. Why should a clinical thermometer not be dropped in boiling water?
Solution: The highest temperature that a clinical thermometer can record is 110°F, whereas the boiling point of water is 212°F.
When the clinical thermometer is dropped in boiling water, it breaks because of the force of expanding mercury in it and becomes useless.
Q11. How does the thermos keep the liquid hot or cold for a long time?
Solution: In a flask, the loss or gain of heat through conduction, convection, and radiation has been reduced to a minimum.
Hence, it can keep a hot liquid hot and a cold liquid cold for a reasonable length of time.
Q12. Ice wrapped in a woolen blanket does not quickly melt away. Why?
Solution: Woollen blanket is a bad conductor of heat. It does not allow the external heat rays to enter.
Therefore, ice does not melt for a considerable length of time.
Q13. Why is water from a hand pump warm in winter and cold in summer?
Solution: In winter, the outside temperature is low compared to the water temperature obtained from the hand pump.
Hence it feels warm. This is because the upper layer of the earth's crust is exposed to the atmosphere at a lower temperature.
Underground water is comparatively at a higher temperature.
In summer, the outside temperature is higher, so the water from the hand pump is felt cooler. 
Q14. Why are mornings and evenings less warm than noon?
Solution: In the mornings and evenings, the sun's rays falling on earth are slanting, so their distance is more.
The earth gets heated up only slightly. At noon, the rays of the sun falling on the ground are nearly vertical, and the distance is also less, resulting in the earth getting heated up considerably.
Hence mornings and evenings are less warm than noon.
Q15. What is the function of a radiator in a motor car?
Solution: After working for some time, the engine of a motor car gets heated.
Water is stored in the radiator, and the same is circulated in the engine to keep it cool. 
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