Although there seems to be much debate about it, steam and water vapour are both invisible to the naked eye, apart from optical refraction effects, when the viewing conditions favour such observation. The distinction is surely based on the conditions that the gaseous form of water is found in. Water vapour is used to describe the gaseous form of water, where its proportion, relative to other gases, is relatively small e.g. water vapour in the atmosphere. In the case of steam, water in its gas-phase is the major constituent. It's when the steam or water vapour encounter suitable conditions, causing condensation to liquid water to occur, that the water becomes visible, because of the optical refraction and light scattering taking place at the surface of the fine droplets of liquid water.
Interestingly, the CRC Handbook of Chemistry and Physics refers to the properties of H2O as attributes of ice, water or steam. The term water vapour is only used when referring to properties associated with atmospheric humidity.
Interestingly, the CRC Handbook of Chemistry and Physics refers to the properties of H2O as attributes of ice, water or steam. The term water vapour is only used when referring to properties associated with atmospheric humidity.