Why is it generally more severe to be burnt by steam at 100°C than by boiling water at 100°C?
Steam contains more latent heat of vaporization.
Both steam and boiling water at 100°C are at the same temperature, so steam is not hotter than the boiling water.
However, when steam at 100°C condenses on the skin, it releases its latent heat of vaporisation (\(\approx 2{,}260 \text{ J/g}\)) in addition to the heat lost when the resulting water cools from 100°C to body temperature.
So the total heat delivered to the skin per gram of steam is roughly:
\(Q_{steam} = m \cdot L_v + m \cdot c \cdot \Delta T\)
whereas for boiling water it is only \(Q_{water} = m \cdot c \cdot \Delta T\). The extra latent heat makes steam burns much more severe.
Hence the correct answer is option 2).
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