UPSC Prelims 2003 · General Studies

Question 22: Science and Technology

Consider the following statements:

1.Steam at 100 °C and boiling water at 100 °C contain the same amount of heat.
2.Latent heat of fusion of ice is equal to the latent heat of vaporization of water.
3.In an air-conditioner, heat is extracted from the room air at the evaporator coils and is rejected out at the condenser coils.

Which of these statements is/are correct?

a1 and 2

b2 and 3

cOnly 2

dOnly 3

AnswerThe answer is (d).

Only statement 3 is correct. An air conditioner works like a refrigerator: the refrigerant absorbs heat from the room air at the evaporator coils and rejects it outside at the condenser coils. Steam at 100 degrees C holds more heat than water at the same temperature because it carries the latent heat of vaporization (about 2,260 kJ per kg), and that is far larger than the latent heat of fusion of ice (about 334 kJ per kg).

1. Steam holds the extra latent heat of vaporization, so it contains more heat than boiling water at 100 degrees C.
2. Latent heat of vaporization (about 2,260 kJ/kg) is much greater than that of fusion (about 334 kJ/kg).
3. An air conditioner takes heat from room air at the evaporator and gives it out at the condenser.

Why each option is right or wrong

(a) Statements 1 and 2 are both false.
(b) Statement 2 is false: vaporization needs far more heat than fusion.
(c) Statement 2 is false, and statement 3 is true, so 'only 2' fails.
(d) Correct: only statement 3 holds.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2003, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.