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When Professor Lavelle was explaining why steam causes severe burns, I think he said that when the steam comes into contact with steam, it takes a longer time for it to become a liquid and releases heat in the process. Can someone explain/clarify this concept?
He said that when steam comes into contact with skin, it condenses since the skin is at a lower temperature. The process of condensation releases energy (as heat) which causes the burn. Since the water is going through a phase change at this point, the temperature of the steam is not getting lower even though energy is being released. This process, and the large amount of heat that it releases, is what makes steam burns so severe.
How I understood this idea is that when water comes into contact with skin, it is already in liquid form and does not change state, therefore it's temperature decreases, releasing energy. When steam comes into contact with the skin, it does change state, from gas to liquid. During this condensation phase change, the temperature does not change, but a huge amount of energy is released. Then, on top of that, once the steam becomes water, it cools down, releasing more energy just like boiling water would.
Think of it this way, when hot water comes in contact with your skin, there will a kind of be a heat energy exchange between the two to bring them to the same temperature, or to equilibrium temperature. Thus the water will cool down and the temperature of skin will rise. Now, as he stated in the water heating curve, steam has a lot more heat energy in it, even at the same temperature as hot water, because all that extra energy is used to break the bonds of liquid water. So if steam has more heat energy, with skin being the same temperature/amount of energy, the equilibrium point for skin and steam would be higher than that of skin and hot water. So with steam, the skin will have to reach a higher temperature, thus itll cause a more severe burn.
Here is a graph that could help supplement what everyone else is saying. Here, you can see that more energy is needed in order to make a gas a gas, so when it comes into contact with skin it releases the more heat it has stored.
- heating-curve.png (6.55 KiB) Viewed 249 times
Essentially 100 degrees Celsius steam burns skin more than 100 degrees Celsius water because the enthalpy of steam is greater. More heat energy is needed to be supplied in order to vaporize water compared to boiling water. Therefore, when steam comes into contact with skin, the amount of heat energy transferred is greater than that of boiling water.
Because the steam undergoes condensation, a phase change, in which it releases a lot of energy. This energy would burn a lot more than just, for example, boiling water dropped on the skin because boiling water would only under go temperature change.
He said that when steam (around 100 degrees C) comes into contact with skin (around 25 degrees Celsius), it releases around 40.7kJ (vaporization energy) and an additional 5kJ (to change from 100 degrees C to 25 degrees C).
Steam causes a more severe burn than that of boiling water even when they are at the same temperature due to the fact that steam holds more heat energy as becoming a vapor from a liquid requires a large amount of heat before it can change phases.
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