Steam

Steam is water in the state of vapour. Although it is popularly supposed that steam is only produced when water is boiling, it can, in point of fact, exist at any temperature. The humldity of the atmosphere is simply due to the presence in it of water vapour or steam. Further, the amount of water vapour present in any space depends only on the temperature, and is independent of the presence of other gases. If we have a vessel, closed by a movable piston and containing nothing else but a little water, that water will begin to evaporate, and continue this action until the space above it is saturated with vapour. At this point the number of molecules which force their way out of the liquid through its surface layer is exactly counter-balanced by the number which return from the space above. There is therefore equilibrium, and as long as the temperature remains constant the pressure of the vapour is unaltered. If we attempt to increase the pressure of the vapour by pushing in the piston (still keeping the temperature the same), we find our efforts unavailing. Some of the steam condenses; it is less in quantity but its pressure persists, and this pressure is known as the vapour pressure. Steam at the vapour pressure, therefore is just on the point of liquefying, and is said to be saturated vapour. The following table gives the vapour pressure of steam at different temperatures :-

Below the freezing-point, it is evaporation of the ice itself which produces water vapour; solid and gas exist together without the presence of the liquid form.

We can give heat to the water in our closed vessel till all is converted to saturated vapour. On applying further heat we cause the temperature of the vapour to rise; it is no longer saturated, but is said to be superheated, and then we can increase the pressure without causing condensation. The steam in fact, behaves like any other gas; pressure and volume vary inversely, diverging but slightly from conformity with Boyle's law. At any one temperature we can plot out a curve showing the connection between the volume and pressure of the steam and such a curve is known as an isothermal. At 100° C. there will be a curved portion, A B, differing very slightly from a hyperbola; then when the pressure is one atmosphere, the vapour is saturated. We cannot increase the pressure further, but steady diminution of volume occurs as more liquid is formed and this is indicated by the straight line B C. Every temperature gives a similar curve, but the length of B C gets less and less as we take higher temperatures, until at one point it vanishes. Ihis point is known as the critical point. Here the temperature, pressnre, and volume have fixed values - not easy to determine experimentally, however. The critical temperature of water is 365 C., and above this temperature it is impossible to liquefy steam by pressure.

If we drew a number of isothernials and connected all at the point B, we should get a curve showing the volumes and vapour pressures of saturated steam at different temperatures. This curve is known as the steam line. It bends up towards the critical point, where it meets a corresponding curve showing the volume of the condensed water formed at different temperatures, and called the water line. It is not possible to show on a small scale the volume of water formed, since the volume of saturated steam at the same temperature is so enormously great compared to it. For instance, 1 lb. of water will occnpy .016 of a cubic foot at 212°, while the steam formed from it will be 26.36 cubic feet. If B C represent the latter quantity, .016 vanishes from view on the same diagram.

Since heat must be given to water to convert some of it into steam, and both remain at the same temperature, it is obvious that this heat has been used up in some other way which a thermometer is unable to indicate. This is known as the latent heat (q v.) of steam, and at 100° C. it requires as much heat to convert a pound of water into a pound of steam as to raise 5.36 lbs. from 0° to 1° C. This is expressed by saying that the latent heat of steam is 5.36. When once the steam is formed, however, the heat required to raise its temperature one degree is less than water would require for a similar purpose, the specific heat of steam being only .4805. If steam is being generated in a closed vessel, the temperatures of water and steam can both rise above 100° C., the pressure of course increasing. At 112° C. we know the pressure is 1 atmosphere; at 120° it is 2 atmospheres; at 134° it is 3, and at 143° it is 4. Hence we note the enormous force exerted on the vessel as heat is applied to it, the force doubling itself twice during a temperature rise of only 430 C. Great care is therefore necessary in dealing with steam under these conditions; otherwise it is liable to burst the vessel in which it is confined, the possibility of such a result being demonstrated with moderate frequency by boiler explosions.