Compared to a burn due to air at 100°C, the burn due to steam at 100°C is
More dangerous due to high latent heat of evaporation
Steam causes the worse burn, because of its high latent heat of vaporisation — option 2.
The reasoning. Both are at the same temperature, so the difference cannot lie in temperature. It lies in the extra energy the steam carries. To turn one gram of water at 100 °C into one gram of steam at 100 °C requires
\(L_{v}=540\ \text{cal/g}=2260\ \text{kJ/kg}\)
and that energy is stored in the steam with no rise in temperature. When steam touches the skin it first condenses, releasing all 540 calories per gram into the skin, and only then does the resulting water cool from 100 °C.
| Contact | Heat given to the skin, per gram |
|---|---|
| Water at 100 °C cooling to 60 °C | About 40 cal |
| Steam at 100 °C condensing, then cooling to 60 °C | 540 + 40 = about 580 cal — roughly fourteen times as much |
| Air at 100 °C | Very little — air has low density and low specific heat, and is a poor conductor |
Why hot air burns least of all. A litre of air at 100 °C contains only about a thousandth of the mass of a litre of water, and air conducts heat poorly. This is exactly why a person can sit in a sauna at 100 °C in comfort but would be scalded instantly by steam at the same temperature.
Where the same fact appears elsewhere. The large latent heat of vaporisation of water explains a great deal: why sweating cools the body so effectively, why a pressure cooker is efficient, why steam is used to carry heat around power stations and factories, and why the water cycle moves so much energy through the atmosphere.
Note the structure of the options : two variables, more or less dangerous and high or low latent heat, give four combinations. Fixing either one settles the answer, and latent heat is the easier — water’s is famously high, which eliminates options 3 and 4 at once.
Hence, the answer is more dangerous due to high latent heat of evaporation.
Similar Questions
Heat transfer that does not require a medium is called
Which of the following is the value of solar constant ?
Important Questions from Heat and Transfer
In summer season, the top and outer surface of the house are painted white because:
Which of the following is a process in which hot, less dense materials rise upward and are replaced by colder, more dense materials?
What is the heat released by a heating filament rated 24 W when it is kept on for 6 seconds?
Space is left between logs of wood when they are burnt, so that:
On a hot summer afternoon, the tent made up of fabric will be preferred as white colour is:
