Surely many had to watch a picture of how an open container with water after a while turns out to be empty. If you cover it with a lid, then the water does not disappear. Everyone knows the reason - the water evaporates. The explanation for this phenomenon is simple: part of the water molecules has a sufficiently high speed of movement in order to leave the liquid. This process of transition of a liquid into a gaseous state is called evaporation.
Another process, namely the conversion of steam into liquid, is called condensation. These two processes, evaporation and condensation, are ongoing: part of the water evaporates, part - condenses. If the volume above the water surface is unlimited, then the evaporation process prevails. Evaporated water is removed, as, for example, occurs above the surface of open water, and the liquid gradually passes into a gaseous state - steam.
But if the amount of free space above the liquid is limited, then a slightly different situation arises. Evaporated water cannot leave this volume, and saturated steam forms above the surface of the water. This is the name of steam in equilibrium, when the amount of evaporated water and condensed steam are equal. Water does not decrease and does not arrive; a state of equilibrium sets in between evaporation and condensation.
Now we know what saturated steam is, and its properties may turn out to be quite interesting for us. From the very beginning, we determined that the amount of free space above the surface of the liquid is limited. Saturated steam formed over it. And if now to reduce this free volume? What will happen? In this case, the established equilibrium between condensation and evaporation is disturbed. The condensation process will begin to prevail, the volume of moisture will increase, and the vapor will decrease.
The vapor pressure at which it is in equilibrium with the liquid is called saturated vapor pressure. If we reduce the amount of free space above water, then the vapor pressure increases. The consequence of this will be the transition of steam into water. With increased pressure, the liquid takes up less space than saturated steam. One more conclusion follows from this: if the temperature is constant, then the saturated vapor pressure is the same for any volume.
There is another option for the behavior of steam - the volume above the surface of the water is reduced, and the transition of steam into liquid does not occur. This means that there is unsaturated vapor above the surface. Subsequently, with a decrease in volume at a constant temperature, steam begins to turn into water - which means that saturated steam has formed. But it was not in vain that the condition was stipulated that everything happens at a constant temperature. There is a certain value at which steam can turn into a liquid.
This value is called critical temperature. The substance remains gas at a temperature above critical, but if it is below critical, then the gas turns into a liquid. Each substance has its own critical temperature value . Two more features of the steam are worth noting: it can be either wet or dry saturated steam. Drops of water are present in the humid, and dry vapor does not contain moisture.
There is also the so-called superheated steam - this is dry steam with a temperature higher than critical. In this case, it is believed that there is no longer any liquid in the closed volume, but only steam is present. Superheated steam is mainly used in engineering and energy. The high temperature of superheated steam allows it to be transported using steam lines and used in steam turbines. Due to the lack of water in superheated steam, the service life of the turbine is increased.
The article discusses what constitutes saturated steam, its types and properties, as well as the process of its formation and transformation into a liquid.