BPSC 70th CCE Re-exam Prelims General Studies · Question 148 of 142

Correct — C, More dangerous due to high latent heat of evaporation. Both the air and the steam are at the same temperature, so temperature cannot be what separates them. What separates them is how much energy each can dump into the skin, and steam carries an enormous extra store that air does not have at all. To turn one kilogram of water at 100 degrees Celsius into one kilogram of steam at 100 degrees Celsius takes about 22.6 lakh joules — 22.6 × 10^5 J/kg, or roughly 540 calories per gram — and the temperature does not rise by a single degree while that energy goes in. That is the latent heat of vaporisation, and it is simply stored in the vapour. When steam touches skin it condenses back to water, and every joule of it is released into the tissue in an instant. Only after that does the resulting water, now at 100 degrees, begin to cool towards body temperature, giving up a further amount of ordinary sensible heat — about 2.6 lakh joules per kilogram in falling from 100 to about 37 degrees. So roughly nine-tenths of the damage a steam burn does comes from the latent heat, before any cooling has even begun. Hot air has no such reservoir. It can only give up sensible heat as it cools, its specific heat capacity is about 1,000 joules per kilogram per degree, and a kilogram of air at these temperatures occupies about a cubic metre — so the energy actually delivered to skin by air at 100 degrees is small enough that a person can stand in a sauna or open a hot oven without injury. The verdict is therefore 'more dangerous', and the reason is that the latent heat is high, not low. Water's latent heat of vaporisation is in fact one of the largest of any common liquid, which is the same property that makes sweating an effective way to cool the body and steam an efficient way to move energy through a power plant.

• (a)Less dangerous due to low latent heat of evaporation — Wrong on both halves. The steam burn is worse, not milder, and water's latent heat of vaporisation is exceptionally high rather than low. This option would be right only in a world where condensing vapour released almost no energy, in which case steam and hot air really would be equivalent.
• (b)More dangerous due to low latent heat of evaporation — The dangerous option, because the verdict is right and only the reason is wrong. A candidate who knows from experience that steam scalds worse than hot air will accept this without reading to the end of the line. But a low latent heat would mean steam carries little extra energy, which is the opposite of the explanation the verdict needs — the danger exists precisely because the latent heat is about 22.6 × 10^5 joules per kilogram.
• (d)Less dangerous due to high latent heat of evaporation — Has the physics right and the conclusion inverted. The latent heat of vaporisation is indeed high, but a high latent heat means more energy is released on condensation, which makes the burn worse, not milder. This is the option for a candidate who recognises the correct physical quantity and then attaches it to the wrong outcome.

Heat added to a substance does one of two things: it raises the temperature, or it changes the state. Energy of the first kind is sensible heat, calculated as mass times specific heat capacity times temperature change. Energy of the second kind is latent heat, absorbed or released at a constant temperature while the substance melts, freezes, boils or condenses, and it is invisible on a thermometer — which is what 'latent', hidden, means. For water the two latent heats are large and unequal: the latent heat of fusion, ice to water at 0 degrees, is about 3.34 × 10^5 joules per kilogram, while the latent heat of vaporisation, water to steam at 100 degrees, is about 22.6 × 10^5 joules per kilogram, nearly seven times as much. Both are among the highest values of any common substance, and both explain everyday facts. Ice cools a drink far more effectively than the same mass of cold water because it must absorb its latent heat of fusion before it can even begin warming. Sweat cools the skin because evaporation carries latent heat away. A steam burn is severe because condensation brings the same latent heat back.

The option set here is a two-by-two grid — the verdict is either 'more dangerous' or 'less dangerous', and the reason is either 'high latent heat' or 'low latent heat' — and only one of the four combinations is coherent. That structure is a gift, because it means the two halves can be settled independently and neither guess needs to depend on the other. Settle the verdict from experience and from the physics: steam scalds, hot air in an oven or a sauna does not, so 'more dangerous'. Settle the reason from the number: 22.6 × 10^5 joules per kilogram is a large quantity, and it is released, not absorbed, when steam condenses, so 'high latent heat'. Combining the two gives option (c) and eliminates the other three at once. The general lesson is worth stating because papers use this construction constantly: when a stem asks for a conclusion and a reason together, check them separately, and never accept an option because its first half is right. Two of the four options here are half-right, and each of them is designed for a candidate who stops reading once the familiar part has been recognised.

• The latent heat of vaporisation of water is about 22.6 × 10^5 joules per kilogram, roughly 540 calories per gram, absorbed or released with no change in temperature
• Steam at 100 °C releases that latent heat into the skin as it condenses, and only then does the resulting water at 100 °C give up further sensible heat as it cools
• Cooling from 100 °C to body temperature releases about 2.6 × 10^5 joules per kilogram of water, so the latent heat accounts for roughly nine-tenths of a steam burn's energy
• Air carries almost no heat by comparison: its specific heat capacity is about 1,000 J per kg per °C, and a kilogram of it occupies about a cubic metre at these temperatures
• The latent heat of fusion of ice, about 3.34 × 10^5 J/kg, is roughly one-seventh of the latent heat of vaporisation — a distinction UPSC has tested directly
• The same high latent heat makes sweating an efficient cooling mechanism and makes steam an efficient working fluid in power plants

• Accepting an option because its verdict is right; option (b) reaches the correct conclusion through a false reason
• Assuming that equal temperature means equal heat content — steam at 100 °C holds far more energy than water at 100 °C
• Confusing the two latent heats: fusion is about 3.34 × 10^5 J/kg and vaporisation about 22.6 × 10^5 J/kg, not the same

BPSC uses the paired conclusion-and-reason option, a compact way of testing whether the candidate can explain a fact rather than merely recall it, and it puts one half-right option directly above the correct one. UPSC prefers a statement list — asking whether steam at 100 °C and water at 100 °C contain the same heat, and whether the two latent heats of water are equal — which tests the same distinction with numbers rather than with a scenario.

• practice — not a real PYQ

  The latent heat of vaporisation of water at 100 °C is approximately

  • (a)3.34 × 10^5 J/kg
  • (b)4.2 × 10^3 J/kg
  • (c)22.6 × 10^5 J/kg
  • (d)1.0 × 10^5 J/kg

  Answer(c) 22.6 × 10^5 J/kg — about 540 cal/g; 3.34 × 10^5 J/kg is the latent heat of fusion of ice, and 4.2 × 10^3 J/kg/°C is close to the specific heat capacity of water.

• practice — not a real PYQ

  While ice is melting at 0 °C, the heat supplied to it

  • (a)Raises its temperature steadily
  • (b)Is used to change its state, with no rise in temperature
  • (c)Is lost entirely to the surroundings
  • (d)Raises the temperature only of the water already formed

  Answer(b) Is used to change its state, with no rise in temperature — that is the latent heat of fusion, absorbed at constant temperature until all the ice has melted.