Why is it generally more severe to be burnt by steam at 100°C than by boiling water at 100°C?
- (a)Steam is hotter than boiling water.
- (b)Steam contains more latent heat of vaporization.
- (c)Steam has a higher specific heat capacity.
- (d)Steam molecules are moving faster.
Answer
Why
Correct — B. Both are at 100°C, so neither is hotter. Steam carries extra energy: the latent heat of vaporisation it absorbed to become steam, about 2260 J for every gram.
On skin, steam first condenses and releases all of that heat, then cools as boiling water would. Water at 100°C has no such store to give up → option (b).
Why the others are wrong
- (a)Steam is hotter than boiling water. — The question fixes both at 100°C, so 'hotter' cannot be the reason. Steam's extra harm comes from energy it holds without any rise in temperature.
- (c)Steam has a higher specific heat capacity. — Steam's specific heat is about 2.0 J/g°C, roughly half of liquid water's 4.18, so the claim is false. Specific heat governs temperature change, not the heat released on condensing.
- (d)Steam molecules are moving faster. — At the same temperature the average kinetic energy of molecules is the same, so steam molecules are not faster on average. Steam's extra energy went into pulling the molecules apart.
Concept
Latent heat is energy absorbed or released while a substance changes state at constant temperature. Water boiling at 100°C keeps taking in heat without getting hotter, because that energy goes into separating the molecules.
Condensation runs the process in reverse. Each gram of steam that turns to water on skin hands back its latent heat, which is why a steam burn is worse than a scald from water at the same temperature.
Per gram, cooling to body temperature (37°C):
water at 100°C gives 4.18 × 63 ≈ 263 J
steam at 100°C gives 2260 + 263 ≈ 2523 J
That is nearly ten times the heat from the same starting temperature.
Key facts
- Water's latent heat of vaporisation at 100°C is about 2260 J/g (22.6 × 10⁵ J/kg).
- Liquid water's specific heat is about 4.18 J/g°C, and steam's about 2.0 J/g°C.
- Condensation releases latent heat and evaporation absorbs it.
Study next
Common traps
- Picking 'Steam is hotter' because steam feels hotter. The question sets both at 100°C.
- Reaching for specific heat. Heat released during a change of state is latent heat, and steam's specific heat is lower than liquid water's anyway.
The same latent heat, absorbed this time as water evaporates, keeps an earthen pot cool at 26 Sep 2024, 16:00, GA Q.15, where 'Environmental water vapour enters the pot through pores' is keyed as the INCORRECT statement.
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