Why is being scalded by boiling steam so much worse for you than being scalded or burnt by a liquid or a solid at the same temperature?

The specific heat capacity of a material measures how much energy is required to change the temperature of that material. The specific heat capacity of water is 4180 joules per kilogram per kelvin, meaning that it requires 4180 joules of energy to raise the temperature of one kilogram of water by one kelvin.

The latent heat of a material is the energy required to change the state of a material without changing the material’s temperature. There are therefore two latent heats: the latent heat of fusion is the energy required to turn a solid to a liquid or vice versa, and the latent heat of vaporisation is the energy required to turn a liquid to a gas or vice versa. For water the latent heat of fusion is 334 000 joules per kilogram and the latent heat of vaporisation is 2 260 000 J/kg.

If a one gram drop of boiling water (at 100°C) falls on skin at a temperature of 35°C then the temperature of the water quickly falls by 65°C. To drop the temperature of one gram of water by 65°C requires a change in energy of 272 joules. Because heat always flows from a hotter body to a colder one* this heat flows into the skin, damaging skin cells as it does.

The situation is different if one gram of boiling steam (still at 100°C) hits the skin. First it has to change state into water, and then cool down just as above. In the process of changing state from a gas at 100°C to water at 100°C it releases a huge amount of energy: an additional 2260 joules when compared with the 272 joules released as it cools. If we assume that the severity of the scald is proportional to the energy released (which is a very reasonable assumption) then a scald with boiling steam does 931% of the damage that a scald with boiling water does.

The graph above shows how the temperature of a 1kg block of ice at −100°C changes as energy is supplied to it. The horizontal sections occur when energy is being absorbed but the temperature of the substance is not changing; this is because the energy is being used to weaken bonds between molecules as the state changes first from solid to liquid and then from liquid to gas. The longer horizontal section in the liquid-gas state change indicates that more energy is required to turn water into steam than is required to turn ice into water. This is reflective of the relative strengths of the intermolecular bonds in solids, liquids and gases. The differing gradients of the sloped sections reflects the fact that the specific heat capacity of water varies with state.†

* More accurately, the net flow of heat is always from a hotter body to a colder one.

† For the sake of simplicity, the specific heat capacity of water in each state has been assumed not to vary.