HEAT: SLC questions and answers

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1. If heat is sup­plied to a solid con­tin­u­ously, it melts.

When solid is heated, the K.E. of the mol­e­cules increases which decreases the bind­ing force of mol­e­cules so if heat is sup­plied to a solid con­tin­u­ously, it melts.

2. A thick glass tum­bler crack when boil­ing water is poured in it.

Glass is a bad con­duc­tor of heat. So when boil­ing water is poured in a thick glass tum­bler, the inner sur­face gets sud­denly expanded whereas the outer sur­face remains same.Hence due to such unequal expan­sion of inner and outer sur­faces, the thick glass tum­bler cracks.

3. Sea breeze occurs at days and land breeze occurs at nights.

Water has max­i­mum spe­cific heat capac­ity and sand has less spe­cific heat capac­ity. Due to it, at days sand of land is heated faster than water and the air above it becomes less dense by its expan­sion. Thus, air above the seas being denser blow toward the land and sea breeze occurs. But at night the sea water cools slowly but sand cools faster, now the air above sea water becomes less dense and that above land is denser. Thus, air over lands moves towards the sea and land breeze occurs.

4. A wet hand­ker­chief is kept on the fore­head of the patient suf­fer­ing from fever.

When a per­son is suf­fer­ing from fever, a hand­ker­chief is kept on the fore­head to lower his/her body tem­per­a­ture. As the wet hand­ker­chief con­tains water, it can absorb a large amount of heat due to its high spe­cific heat capac­ity. As a result, the tem­per­a­ture (i.e. high fever) reduces. Hence a wet hand­ker­chief is kept on the fore­head of the patient suf­fer­ing from fever.

5. The metal anti­mony behaves like water.

The meta anti­mony expands on freez­ing and con­tracts at first when warmed above its melt­ing point. Due to such expan­sion, it makes sharp cast­ings, Hence it behaves like water.

6. Water  in the earthen pot is cooler than that in a metal pot.

Earthen pot is porous. Due to cap­il­lary action, small amount of water of the pot rises to the outer sur­face of pot. The heat energy absorbed from envi­ron­ment evap­o­rates water from the sur­face of the pot. The tem­per­a­ture of water in the pot doesn’t get heat from the envi­ron­ment and remains cold.

7. When we get out of bed in the very cold morn­ing we feel air of the room cold but when we come back stay­ing out­side for some time, we felt air in the room warm.

Heat always flows from high tem­per­a­ture to low tem­per­a­ture. The tem­per­a­ture of our body is higher than that of air tem­per­a­ture of the room, So heat flows from our body to the air of room and we feel cold. But when we came back to the same room after stay­ing out­side for some time, our body tem­per­a­ture is lower than that of air tem­per­a­ture of the room. So heat flows from the air to our body and we feel warm.

8. Fish can sur­vive under a pond where the sur­face is even cov­ered with ice.

Water has max­i­mum den­sity at  due to the anom­alous expan­sion of water and is found at the bot­tom of the pond. There are other lay­ers of water of , ,  and  respec­tively above water of . As ice is insu­la­tor of heat so it does not allow the heat of pond water to escape out. Hence, fish can sur­vive under a pond where the sur­face is even cov­ered with ice.

9. The tem­per­a­ture of water at the bot­tom of a pond is  when its sur­face water freezes.

The tem­per­a­ture of water at the bot­tom of a pond is  when its sur­face water freezes due to the high­est den­sity, that cause water at  denser and sinks at the bottom.

10.A bot­tle com­pletely filled with water gets burst when it is kept in refrigerator.

OR

       Soft drink bot­tle (like coke) are not com­pletely filled.

A bot­tle com­pletely filled with water is cooled at  in refrig­er­a­tor, it expands due to anom­alous expan­sion of water i.e. ice has large vol­ume. The increase in vol­ume gives very large force on the bot­tle and gets burst.

11. The water pipes filled with water often burst in winter.

Water pipes burst in win­ter sea­son due to anom­alous expan­sion of water. When water is cooled from  — , it expands and gives out­ward pres­sure. Hence, the water pipes filled with water often burst in winter.

12. If a beaker com­pletely filled with water of  is heated or cooled, water overflows.

If a beaker com­pletely filled with water of  is heated or cooled, water over­flows because of anom­alous expan­sion of water. When water at  is heated or cooled its vol­ume increases.

13. The desert is very hot at day and very cold at night.

Sand gets heated quickly due to the sun’s heat dur­ing day time and gets cooler soon in the absence of sun’s heat at night due to less spe­cific heat capac­ity of sand so the desert is very hot at day and very cold at night.

14. The cli­mate nearby the oceans remains pleas­ant through­out the year.

The spe­cific heat capac­ity of sand is five times lesser than that of water so the tem­per­a­ture of the land remains higher than that of water at day and heat flows from land to the ocean and occurs vice-versa at night. Hence, the cli­mate nearby the oceans remain pleas­ant through­out the year.

15. If the sand and water of equal weight are heated, the tem­per­a­ture of sand will be higher.

If the sand and water of equal weight are hated, the tem­per­a­ture of sand will be higher because it has less spe­cific heat capac­ity as sp. heat capac­ity of sub­stance is inversely pro­por­tional to the increas­ing temperature.

16. Milk boils faster than water.

Water has very high spe­cific heat capac­ity    as com­pared with other liq­uids (e.g. oil, water etc.) so it takes a longer time to boil as it takes larger amount of heat to boil.

17. Steel chair feel colder than the wooden chair dur­ing the win­ter season.

Dur­ing win­ter sea­son, when the tem­per­a­ture fall down the amount of heat lost by the steel chair will be greater than the wooden chair because the spe­cific heat capac­ity of steel or iron is greater than the wood.

18. Equal mass of water and oil heated for some time, when cools down water cools slowly than oil.

Equal mass of water and oil heated for some time, when cools down water cools slowly than oil because water has very high spe­cific heat capac­ity  as com­pared with other liq­uids (e.g. oil) so it takes a long time to cool.

19. Ice is cov­ered with saw dust pre­vents the ice to melt.

Since the saw dust is a poor con­duc­tor of heat and there­fore it pre­vents the heat trans­fer from air to ice so ice is cov­ered with saw dust pre­vents the ice to melt.

20. The water is used in the hot water bag instead of oil.

The water is used in the hot water bag instead of oil because water has very high spe­cific heat capac­ity  as com­pared with other liq­uids (e.g. oil) and it can store a large amount of heat energy for a longer period of time in the water bag. So, the water is used in the hot water bag instead of oil.

21. Water is used in cool­ing the engines of motor car.

Water has capac­ity to absorb a large amount of heat from the heat engines as it has very high spe­cific heat capac­ity  as com­pared with other liq­uids. So, water is used in cool­ing the engines of motor car.

22. Well water is warmer in the morn­ing dur­ing win­ter season.

The spe­cific heat capac­ity of soil is very less as com­pared to that of water so dur­ing win­ter night, the tem­per­a­ture of soil decreases very fast than that of water. Hence, well water is warmer in the morn­ing dur­ing win­ter season.

23. We wear warm clothes in winter.

Dur­ing win­ter, the tem­per­a­ture of sur­round­ing is very low as com­pared to our body tem­per­a­ture. So, more amount of heat is lost from our body to the sur­round­ing and we feel very cold. Hence we wear warm clothes in win­ter to pre­vent the heat loss from the body as the woolen clothes are also the bad con­duc­tor of heat.

24. A new quilt is felt warmer than an old one.

A new quilt encloses more spaces that get filled with more air as com­pared to that of the old quilt. Since, air is bad con­duc­tor of heat, it does not allow the heat to flow from the body to the sur­round­ing. As a result, a new quilt is felt warmer than the old one.

25. Dur­ing the win­ter, the ani­mals curl into a ball.

The amount of heat loss by a body depends upon its area. When the ani­mals curl into a ball, the sur­face area of their body will be reduced. As a result, the heat loss to the sur­round­ing will be reduced. This helps them to remain warm in the winter.

26. We sweat in summer.

In sum­mer, the atmos­pheric tem­per­a­ture goes up higher than our nor­mal body tem­per­a­ture () to main­tain our body tem­per­a­ture fixed at () our body loses the excess heat by excret­ing sweat. So, we sweat in summer.