Mock Test

Heat and Thermodynamics part-(2)

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Heat and Thermodynamics part-(2)

Total Questions: 44

21. The cooling by a desert cooler is based on - [R.A.S./R.T.S (PRE) 2012]

Correct Answer: (c) evaporating cooling.
Solution:The cooling by a desert cooler is based on evaporate cooling these are the cooler known as swamp coolers.

22. Cooling is done by - [47th B.P.S.C. (PRE)]

Correct Answer: (b) release of compressed gas.
Solution:

• This principle is used in refrigeration and air conditioning systems.

• A gas (refrigerant) is compressed, which raises its temperature.

• This hot, compressed gas is then cooled and allowed to expand rapidly.

• This rapid expansion causes the gas to cool significantly, absorbing heat from its surroundings, which provides the cooling effect. Most of the air conditioner uses compressed gas which can cool the room or other places.

23. In which of the following condition the wet cloths dry earlier ? [U.P.P.C.S.(pre) 2008]

Correct Answer: (d) 20% RH, 60°C temperature.
Solution:
  • 20% RH (Relative Humidity) indicates very dry air, which promotes faster evaporation.
  • Higher temperature increases the kinetic energy of water molecules, making them evaporate faster.

24. Which of the following changes occur when salt is added into the water? [R.A.S./R.T.S (PRE) 1992]

Correct Answer: (a) boiling point is increased and the freezing point decreased.
Solution:• When salt is added to water it forms a solution. The presence of the salt ions disrupts the ability of water molecules to escape into the vapor phase, which is necessary for boiling. So need to provide more heat to water for boil. This means the boiling point increases.

•Similarly, the presence of salt ions disrupts the formation of the regular crystal lattice that occurs when water freezes. This means you need to lower the temperature even further for the water to freeze. Therefore, the freezing point decreases.

25. On adding common salt to water the boiling point and freeze point of water will : [U.P.P.C.S.(MAINS) 2006]

Correct Answer: (d) increase and decrease respectively.
Solution:If salt is added to water then the boiling point of water will increase this happens as the boiling point is the temperature at which the vapour pressure of solvent becomes equal to the external atmospheric pressure similarly it also decreases the freezing point which in turn will interface with the frozen solution crystal structure that means the temperature will have to be colder to overcome it and freeze the mixture.

26. An assertion and its reason is given below [Uttarakhand P.C.S. (PRE) 2012]

Assertion (A) : Salt is used for cooking the food at places on higher from sea level .
Reason (R) : Temperature is low in higher places which one among these if right answer ?

Correct Answer: (b) Both (A) and (R) are true, and (R) is not the correct explanation of (A).
Solution:At high altitude the temperature pressure is low so water will boil at a lower temperature because the boiling of water is very lower at high altitude. It takes longer to cooked at higher altitude than at a sea level.

Adding the small amount of salt in water that will cause the water to boil at a slightly higher temperature which can be helpful while in cooking especially at high altitude .Lower the pressure at higher altitude causes the temperature to be colder.

27. To keep drinks at the same temperature for quite sometime the walls of thermos are coated with: [R.A.S/R.T.S (PRE) 1996]

Correct Answer: (c) silver layer.
Solution:The glass or a steel which is used in thermos bottle is coated with a silver layer to keep the drink same temperature for some time .

28. It is more comfortable to wear white cloths in summer because: [U.P.P.C.S.(pre) 2007]

Correct Answer: (a) they reflect all the heat failing on them.
Solution:We use white cloths in summer because white cloths are good reflector of heat thus we do not feel warm. In winter gray or dull coloured cloths are used because gray or dull coloured cloths are good absorber of heat.

29. White cloths are cooler than the black cloths because : [U.P.R.O/A.R.O.(MAINS) 2016]

Correct Answer: (d) they reflect the whole light.
Solution:We use white cloths in summer because white cloths are good reflector of heat thus we do not feel warm. In winter gray or dull coloured cloths are used because gray or dull coloured cloths are good absorber of heat.

30. An assertion and its reason is given below: [Uttarakhand P.C.S. (PRE) 2012]

Assertion (A) : we prefer the white dress in winter.
Reason (R) : white dresses are good reflector of heat.
which one among these is the right answer.

Correct Answer: (d) (A) is false, but (R) is true.
Solution:We use white cloths in summer because white cloths are good reflector of heat thus we do not feel warm. In winter gray or dull coloured cloths are used because gray or dull coloured cloths are good absorber of heat.

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