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Difficulty: MediumThermodynamics and Heat

In an occupational safety training session, workers are informed that exposure to steam at 100∘C100^\circ\text{C} causes significantly more severe burns than exposure to liquid water at 100∘C100^\circ\text{C}. Which thermodynamic factor directly accounts for this difference?

  1. A
    A higher average molecular kinetic energy in steam molecules than in liquid water molecules
  2. The substantial release of latent heat as steam condenses into liquid upon contact with the skinAnswer
  3. C
    A faster transfer of thermal energy driven exclusively by electromagnetic radiation
  4. D
    A rapid exothermic chemical reaction occurring between water vapor and skin tissue

Answer

The substantial release of latent heat as steam condenses into liquid upon contact with the skin
When steam at 100∘C100^\circ\text{C} comes in contact with cooler skin, it undergoes a phase change into liquid water at 100∘C100^\circ\text{C}. During this condensation process, it releases its latent heat of vaporization (approximately 2260 J/g2{}260\text{ J/g}) directly onto the skin before the resulting water even begins to cool. By contrast, boiling liquid water at 100∘C100^\circ\text{C} only releases sensible heat as its temperature drops (q=mcΔTq = mc\Delta T), yielding roughly 264 J/g264\text{ J/g} when cooling to body temperature. The additional latent heat released by steam transfers nearly ten times as much total energy to the skin, causing far more destructive burns.

Step-by-Step Solution

1
Analyze the thermal energy transferred by liquid water at 100∘C100^\circ\text{C}
Liquid water at 100∘C100^\circ\text{C} transfers sensible heat only as it cools down to body temperature (37∘C37^\circ\text{C}), which equals approximately 264 J/g264\text{ J/g}.
Liquid water does not undergo a phase change when contacting skin.
2
Analyze the thermal energy transferred by steam at 100∘C100^\circ\text{C}
Steam at 100∘C100^\circ\text{C} must first condense into liquid water at 100∘C100^\circ\text{C}, releasing approximately 2260 J/g2{}260\text{ J/g} of latent heat of condensation, and then release another 264 J/g264\text{ J/g} of sensible heat as the liquid cools.
Phase change from vapor to liquid requires the release of latent heat of vaporization.
3
Compare total energy delivered to the tissue
Steam delivers nearly ten times more thermal energy per gram than boiling water at the same temperature.
The large latent heat of condensation accounts directly for the greater severity of the burn.

Key Concept

Latent heat of phase change and heat transfer during condensation
Estimated Time:1m 0s
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Topics

Thermodynamics and Heat

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