Which of the following statements about latent heat for a given substance is/are correct ? 1. It is fixed at a given temperature. 2. It depends upon the temperature and volume. 3. It is independent of temperature and volume. 4. It depends on the temperature but independent of volume. Select the correct answer using the code given below :
- (a)2
- (b)1 and 3
- (c)4 only
- (d)1 and 4
Correct — D, 1 and 4. Specific latent heat is the heat needed to change unit mass of a substance from one phase to another with no change in temperature. Two things follow. Statement 4 is right because the value is a function of the temperature at which the change of phase happens and of nothing else that matters here — water's latent heat of vaporisation is about 2,260 kJ per kilogram at 100 °C but noticeably lower at higher temperatures, falling to zero at the critical point, while doubling or halving the container's volume leaves the value per kilogram untouched. Statement 1 is simply the other side of the same coin: once the temperature is named, the value is pinned down. Statements 1 and 4 therefore agree with each other and both are correct, which is exactly what option (d) says.
- (a)2 — Statement 2 adds volume to the list of things latent heat depends on. Specific latent heat is defined per unit mass, so the size of the sample and the volume it occupies do not enter the number at all.
- (b)1 and 3 — Statement 3 claims independence of temperature, which contradicts statement 1's 'at a given temperature' and is wrong on the physics. Water's latent heat of vaporisation is not the same at 100 °C as it is at 150 °C, so this pairing is internally inconsistent.
- (c)4 only — Statement 4 is correct, but so is statement 1 — the two say the same thing in different words, so a code that keeps one and drops the other cannot be right. This is the nearest miss in the set and the option most often chosen by candidates who stop reading after finding a true statement.
During a change of phase, heat supplied does not raise the temperature; it goes into breaking the bonds that hold the molecules in the denser phase. That is why a thermometer in melting ice stays at 0 °C and one in boiling water stays at 100 °C however hard the flame burns. The heat absorbed per unit mass in that process is the specific latent heat — of fusion for solid to liquid, of vaporisation for liquid to gas. For water the two values are about 336 kJ per kilogram and about 2,260 kJ per kilogram, and the large gap between them is why steam at 100 °C scalds far worse than boiling water at the same temperature.
The safe way through a four-statement code item is to test the statements against each other before testing them against the physics, because a well-set item usually contains a contradictory pair. Here statements 2, 3 and 4 are three rival claims about the same two variables, so at most one of them can be true; and statement 1 is compatible only with the one that keeps temperature in the picture. That reasoning alone narrows the field to a code containing 1 and 4. On the physics, the point to hold on to is that specific latent heat is a per-kilogram quantity, so it can only depend on the state of the substance, not on how much of it there is or what volume it fills. A related exam favourite rests on the same idea: because latent heat is absorbed with no temperature rise, steam and boiling water at 100 °C do not contain the same amount of heat, a statement UPSC marked false in 2003.
- Specific latent heat is the heat per unit mass needed to change phase at constant temperature.
- For water, latent heat of fusion is about 336 kJ/kg and of vaporisation about 2,260 kJ/kg.
- The value varies with the temperature of the phase change and falls to zero at the critical point.
- Being a per-unit-mass quantity, it does not depend on the volume or the size of the sample.
- Steam at 100 °C carries far more energy than boiling water at 100 °C, by exactly the latent heat of vaporisation.
Statements 2, 3 and 4 are rival claims about the same pair of variables, so only one of them can survive.
- Stopping at the first true statement instead of checking whether a second one says the same thing.
- Assuming latent heat is a fixed universal constant for a substance regardless of the temperature of the phase change.
- Letting the size of the sample creep into a quantity that is defined per unit mass.
NDA sets latent heat either as a definition item or, as here, as a code question in which two statements are paraphrases of each other and must both be selected.
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?
- (a) 1 and 2
- (b) 2 and 3
- (c) Only 2
- (d) Only 3
Answer(d) Only 3
UPSC marked both latent-heat statements false — steam holds more energy than boiling water, and fusion and vaporisation values are far apart. The same understanding that answers this NDA item.
The specific latent heat of vaporization of a substance is the quantity of heat needed to change unit mass from
- (a) liquid to vapour with a change of temperature
- (b) liquid to vapour without a change of temperature
- (c) vapour to liquid without a change of temperature
- (d) vapour to liquid with a change of temperature
Answer(b) liquid to vapour without a change of temperature
The definition this 2018 item tests indirectly, keyed four years later. The words 'unit mass' are precisely why statement 2 of the 2018 item, which drags volume in, is false.
The amount of heat required to change a liquid to gaseous state without any change in temperature is known as
- (a) specific heat capacity
- (b) mechanical equivalent of heat
- (c) latent heat of vaporization
- (d) quenching
Answer(c) latent heat of vaporization
The same definition set one year before this paper, and a reminder that NDA tests latent heat in almost every sitting.
- practice — not a real PYQ
Steam at 100 °C causes a more severe burn than water at 100 °C because
- (a)steam is at a higher temperature
- (b)steam gives up its latent heat of vaporisation on condensing on the skin
- (c)steam has a higher specific heat capacity
- (d)steam conducts heat better than water
Answer(b) steam gives up its latent heat of vaporisation on condensing on the skin — about 2,260 kJ per kilogram released before it even begins to cool.
- practice — not a real PYQ
While ice at 0 °C is melting, the heat supplied to it
- (a)raises the temperature of the ice
- (b)is used to break the bonds holding the solid together, at constant temperature
- (c)escapes entirely to the surroundings
- (d)increases the specific heat capacity of the water formed
Answer(b) is used to break the bonds holding the solid together, at constant temperature — which is what latent heat means.