Compared to a burn due to air at 100°C, the burn due to steam at 100°C is

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UnlockIAS · Accepted Answer

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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2025 Q148 — Science & Technology PYQ with Answer

State PCS 2025 Q148 — Science & Technology PYQ with Answer