A substance in the gas phase that is at a lower temperature than the critical point is called steam. Due to this characteristic, the vapor can be condensed into a liquid or a solid by increasing its pressure without reducing the temperature.
That is, steam is a gas that can condense at a constant temperature, increasing the pressure. On the other hand, to convert a non-vaporous gas to liquid, it is not enough to increase the pressure, but the temperature will have to be lowered.
For example, water has a critical temperature of 374 ° C (647 kelvin), which is the highest temperature at which liquid water can exist.
A vapor can coexist with a liquid or solid. When the vapor coexists with a liquid or a solid, the two phases will be in equilibrium, and the gas pressure will be equal to the vapor pressure balance of the liquid (or solid).
The vapor state is the state in which a gas is when it is below the critical temperature. In the gas state the molecules that form it do not react with each other forming molecular bonds, but tend to repel each other. When repelling, the molecules adopt the shape and volume of the container that contains them and tend to separate. That is, they expand and tend to occupy all available volume.
In the graph, steam is the so-called gaseous phase, closed, by the vertical line that represents the critical temperature and the blue curves (vaporization curve) and red sublimation curve, which represent the ways in which a liquid or solid matter is turns into steam Superheated steam is the gas that is above its critical temperature but below its critical pressure.
What is the difference between gas and steam?
From a point of physics gas and steam is the same. In a gas, the weak interaction of its molecules does not get to form molecular bonds. However, steam is commonly used to talk about substances in a gaseous state that are liquid under ambient conditions.
Normally, the word vapor value, if it is not specified what substance it is, usually refers to water vapor. Water vapor is the gaseous phase of the water molecule that is liquid at room temperature.
Water vapor is a gas that is produced by boiling when the water is heated to 100 ° C and an atmosphere of pressure. Under these pressure conditions, above 100 degrees Celsius, pure water is in the gas state; It is water vapor.
How important is steam?
Water vapor in the atmosphere is a greenhouse gas as is carbon dioxide. Some of the solar radiation that enters the atmosphere, bounces against the Earth's surface and goes outside again. However, greenhouse gases retain this heat contributing to global warming.
Atmospheric water vapor is part of the hydrological cycle, a closed system of water circulation from the oceans and continents to the atmosphere and vice versa, in a continuous cycle of evaporation, perspiration, condensation and precipitation. Its concentration in the atmosphere is very variable in space and time depending on weather conditions.
Water vapor is the greenhouse gas with the highest concentration in the atmosphere (on average 0.33% up to 4% in certain periods) and is the one that contributes most to the greenhouse effect (with an estimated contribution of 70%, which reaches 98% if the effect created by clouds is also considered). The same theory of the greenhouse effect was born in the early 1800s precisely to explain the determining role of water vapor in the atmosphere.
In the atmosphere, water molecules capture the heat radiated from the Earth by branching it in all directions, thus heating the Earth's surface before being radiated back into space.