Remember steam is just a medium for getting energy from one place to another, so that the energy can be converted to work. As I remember from my college days; I#m going to get a rollocking from JJB, I suspect! Hot water at 100degrees C contains roughly 420 kiloJoules of energy. After you've added the energy to get to this point, you've got hot water just beginning to boil; the liquid water is turning into a vapour (steam). As you add more and more heat, a greater proportion of the water is turned to steam. If I recall the amount of heat required to turn a mass of water into an equal mass of dry steam is around 2.2 MegaJoules, at 100 degrees C, and it's this latent heat of vaporisation which makes steam so useful. If you use the steam to move a piston in a cylinder, some of the energy is given up as work; if you cut off the steam inlet at a point before the end of the piston stroke, more energy is lost from the steam as the pressure drops. Obviously, the pressure cannot go below atmospheric, or the steam won't go up the chimney. As the steam expands, it cools, and as it does so some of it will condense back to water, if it's not superheated. It's the very poor expansion ratio (1 to 5 is about the maximum you can get away with, IIRC) that gives the simple steam reciprocating engine/steam locomotive boiler combination its low efficiency. In a petrol engine, you can get away with 12 to 1 (?), and for a diesel it's even higher.