Question

If you get boiling water at \(100^{\circ} \mathrm{C}\) on your skin, it burns. If you get \(100^{\circ} \mathrm{C}\) steam on your skin, it burns much more severely. Explain why this is so.

Short answer

Steam causes more severe burns because it releases additional latent heat upon condensation.

Step-by-step solution

Understanding Heat Transfer

When water boils at 100°C, it turns into steam. Both the water and steam are at the same temperature, but they transfer heat differently upon contact with the skin.

Latent Heat of Vaporization

Steam releases additional energy in the form of latent heat of vaporization when it condenses on the skin. This energy is much greater, about 2260 kJ/kg, compared to the energy from merely cooling down by 1 degree in boiling water.

Energy Transfer Comparison

Upon contact with the skin, boiling water at 100°C only transfers its thermal energy to cool from 100°C to body temperature. However, steam at 100°C releases its latent heat first during condensation and then its thermal energy as it cools.

Conclusion

The severe burns caused by steam are due to the extra energy released during the phase change from gas (steam) to liquid (water), which is not present in boiling water.

Key concepts

Latent Heat of Vaporization

The concept of the Latent Heat of Vaporization is crucial to understanding why steam burns are more severe than those caused by boiling water. The latent heat of vaporization refers to the additional energy required to transform a liquid into a gas at a constant temperature.
When steam comes into contact with skin, it undergoes a phase change from gas to liquid, a process known as condensation. During this process, steam releases a significant amount of energy.

• This energy is called the latent heat of vaporization.
• It is roughly 2260 kJ/kg for water.

This means that when steam condenses on the skin, it gives off this large amount of energy, in addition to the energy from its temperature, which can result in more severe burns compared to boiling water.
In contrast, boiling water at 100°C does not have this extra latent heat to release, making steam potentially more dangerous.

Phase Change

Phase changes occur when a substance transitions between different states of matter: solid, liquid, and gas. We often talk about water boiling or freezing, which involve phase changes. A relevant phase change in this discussion is when steam (gas) changes back into water (liquid).
Phase changes, like the condensation of steam, are associated with significant energy transfers.

• Steam at 100°C changing back to liquid gives off latent heat.
• This energy transfer occurs without any change in temperature.

When steam condenses and transitions back to a liquid state on the skin, it releases the latent heat into the skin tissues, making steam burns potentially more intense.
That's why understanding phase changes can help explain the difference in the severity of steam and boiling water burns.

Thermal Energy

Thermal energy is the energy that comes from heat. It's associated with the motion of molecules within a substance. When you touch an object, like boiling water or steam, the thermal energy is transferred to your skin.
In the case of boiling water, the thermal energy is transferred as the water cools from 100°C to body temperature, leading to a burn. Steam, however, poses a greater risk:

• It first releases latent heat during condensation.
• Then it transfers thermal energy as it cools to body temperature.

This dual release of energy - from both latent heat and subsequent cooling - results in a much more intense transfer of energy.
Thus, understanding the dynamics of thermal energy transfer helps us appreciate why steam can cause more severe burns than boiling water.