Question

86\. Water at 100 \({ }^{\circ} \mathrm{C}\) (its normal boiling point) could certainly give you a bad burn if it were spilled on the skin, but steam at 100 \({ }^{\circ} \mathrm{C}\) could give you a much worse burn. Explain.

Short answer

In conclusion, steam at 100°C can cause a much worse burn than water at the same temperature because of the additional latent heat of vaporization it contains. When steam comes into contact with the skin, it releases this extra heat energy, leading to more severe burns compared to liquid water at the same temperature.

Step-by-step solution

Understand the heat energy involved

When water is heated, its molecules gain energy and move more rapidly. At 100 °C under normal atmospheric pressure, water undergoes a phase change from liquid to gas (steam). During this process, the water molecules absorb additional heat energy without increasing their temperature – this heat energy is called latent heat. The latent heat of vaporization for water is approximately 2260 kJ/kg, which represents the additional heat energy that steam at 100 °C contains compared to liquid water at the same temperature.

Steam condensing on the skin

When steam at 100 °C comes into contact with your skin, it will begin to condense and release the latent heat of vaporization back into the surrounding environment. Since your skin is much cooler than 100 °C, this process happens rapidly and the steam releases a significant amount of heat energy onto your skin.

Comparing heat transfer from water and steam

If you come into contact with water at 100 °C, there is no phase change involved and thus no additional latent heat released onto your skin. The heat transfer from water at 100 °C to your skin depends mainly on the heat capacity and the mass of the water in contact with your skin. Comparatively, when steam at 100 °C comes into contact with your skin, the amount of heat transferred is much greater due to the additional latent heat of vaporization being released. This extra heat energy can cause more severe burns.

Conclusion

In conclusion, steam at 100 °C can cause a much worse burn than water at the same temperature because of the additional latent heat of vaporization it contains. When steam comes into contact with the skin, it releases this extra heat energy, leading to more severe burns compared to liquid water at the same temperature.

Key concepts

Heat Transfer

Heat transfer is the movement of thermal energy from a warmer object to a cooler one. This process occurs in several ways, including conduction, convection, and radiation.
Conduction happens when heat moves through a solid, from molecule to molecule, like when a metal spoon heats up in hot soup.
Convection is the transfer of heat by the movement of fluids like air or water. For example, warm air rising and cool air sinking in a room demonstrates convection. Understanding heat transfer is crucial because it determines how temperature changes occur when substances at different temperatures come into contact. In the case of water and steam both at 100 °C, the heat transfer process becomes more significant because of the involvement of phase change, which we will explore further in the next section.

Phase Change

Phase change is a transition of matter from one state to another, such as from solid to liquid or liquid to gas. Importantly, this change doesn't involve a change in temperature, even though heat energy is required or released. In the context of steam and water at the boiling point, steam's transition from a gas to a liquid when it meets cooler skin involves releasing a substantial amount of energy known as latent heat of vaporization.
During this phase change, steam at 100°C becomes water at the same temperature, yet it releases much more heat compared to water simply staying as a liquid.
This makes steam more dangerous at the same temperature because of the sudden and additional energy transfer involved in condensation.

Thermodynamics

Thermodynamics, the study of heat and energy transfer, plays a vital role in understanding why steam burns worse than water at 100°C. One key principle involved is the first law of thermodynamics, which states energy is conserved.
This means the latent heat of vaporization stored in steam is transferred as heat when steam condenses. Another important concept is specific heat, the amount of heat required to change the temperature of a substance. Water has a high specific heat, so it absorbs and holds heat well. However, the latent heat of vaporization adds an extra layer. Understanding these principles helps clarify why steam transferring its hidden energy during condensation can lead to a quicker and more severe burn compared to water.