One of word meanings ""steam"" - substance in gaseous state while the gas phase is in balance with its liquid or firm phases of the same substance. To observe process, it is enough to put a pan with water on fire. The word ""couples"" has also the second value. This field which during the period of vegetation is not occupied with crops and is kept clean.
Molecules of substance are not mobile at all. When substance is in strong aggregate state, they move rather slowly. At temperature increase the motion of the molecules accelerates, and some of them come off bulk. You observed this process when cooking more than once. Of course, water evaporates also without heating, but this process is well noticeable if a reservoir big or if you left a vessel with water without long time. Along with evaporation there is an opposite process - condensation. In this case molecules come back. It is possible to observe it, having put water to be boiled in the closed ware. Having slightly opened at some point a cover, you will see that it is covered with droplets. It means that molecules came off too much, steam became saturated, that is it when its concentration became greatest possible at this temperature and this pressure. Of course, in a case with a pan of purity of an experiment it will not be possible to achieve because it is not tight and a part of molecules by all means will be removed from a system. During steam formation temperature of all system remains invariable until until all liquid evaporates. The gas having the same chemical formula, but significantly bigger volume is formed. It has the same temperature. Only at full evaporation temperature starts over again increasing therefore superheated steam turns out. At different substances steam formation temperature different. At the same time it will be different and with different pressure. For example, with a critical pressure water evaporates not at 100º, and at 0ºС. At the same time phases of substance are not divided. This property found application in boilers. Use of steam in the industry caused at the time the real revolution. Studying its properties began in France in the middle of the nineteenth century. Emergence of engines and steamships allowed to receive new communication networks, and emergence of steam turbines caused rapid development of power. In steam devices both saturated, and superheated steam was used. The second received bigger distribution as its efficiency is higher. The power plants working together are used and now. In the industry also other method of steam formation - sublimation found application. It is called still sublimation. In this case solid substance turns into gaseous state at once. It is possible practically with any substances at certain temperatures and pressure. The method of sublimation is applied to purification of metals. Substance is turned into the gas, impurity having other chemical properties, at the same time are removed. After that from the cleaned particles of substance grow up clean crystals. The method of sublimation is applied also in the space industry to providing thermal insulation of aircraft during descent.