What are the differences between gas and steam?

Answer 1

Steam is the gaseous form of water, but it really can only be considered to be fully gaseous when it is in a dry state, as at temperatures below the dry saturated steam line, it will contain droplets of water. Thus at atmospheric pressure it needs to be above 100 celsius to be dry, the required temperature for dryness rises with pressure. The Critical point for steam is at 374 celsius and 220 bar pressure, above this temperature it is supercritical and cannot be liquefied by any further increase in pressure. Thus in summary, steam above the boiling point at a particular pressure behaves as a gas, and above 374 celsius it is a gas whatever the pressure.

Most gases like oxygen and nitrogen have liquid phases at much lower temperatures, ie well below any normal atmospheric condition experienced even in extreme low weather conditions, so we regard them as always gaseous, but actually they also have liquid/gaseous interface conditions, we just don't experience them in practice except in very low temperature work.

In popular terminology 'steam' is what you see rising above the surface of hot water or from the spout of a kettle, but the fact that it is visible is due to the presence of water droplets and in this condition it is not gaseous steam as described above.

Answer 2

Gas is a state of matter in which the particles have high energy and move freely, while steam specifically refers to the gaseous state of water. Gas can be any substance in a gaseous state, while steam is water vapor resulting from boiling water. Both are invisible and can fill their containers.