If you put your hand in steam, you can get a bad burn. Why? What enthalpy term would you use to describe the event?

Question

wolfe8 · Answer

I think it is: the heat capacity of steam is high.

doc.brown · Answer

Looking at a table of Enthalpies of Formation, it says that H2O as a gas gives off 241.8kJ of energy when it is formed.  You can watch this happen when the space shuttle mixes hydrogen and oxygen together.  Lots of heat coming out.
To make H2O a liquid there has to be a further release of energy.
Enthalpy of Formation of H2O as a liquid is -285.8, which means it gives off 285.8kJ of energy when built from the ground up.  To change gas to liquid you have to get rid of the difference in energy.
-285.8 - (-241.8) = -44kJ
The steam, or water as a gas, touches your skin and transfers 44kJ per mole of gas to your skin, which has water in it.  The 44kJ of energy is exactly the amount of energy needed to heat the water in your skin to boiling.  The boiling water expands and destroys cells from the inside out.
That transfer of energy is explained by Hess's Law.