In
physical chemistry, and in
engineering,
steam refers to vaporized water. It is a pure, completely invisible
gas (for
mist see below). At standard atmospheric pressure, pure steam (unmixed with air, but in equilibrium with liquid water) occupies about 1,600 times the volume of liquid water. In the atmosphere, the
partial pressure of water is much lower than 1 atm, therefore gaseous water can exist at temperatures much lower than 100 C (see
water vapor and
humidity).
In common speech, steam most often refers to the white
mist that condenses above boiling water as the hot vapor ("steam" in the first sense) mixes with the cooler air. This mist is made of tiny droplets of liquid water, not gaseous water, so it is no longer technically steam. In the spout of a steaming kettle, the spot where there is no condensed water vapor, where there appears to be nothing there, is steam.