Evaporate liquids at all temperatures

The Evaporate is the transition of a liquid from the liquid to the gaseous aggregate state (phase transition). If a liquid is placed in an evacuated vessel, it evaporates until an equilibrium has been established between the liquid and the vapor phase. The pressure that then prevails in the vessel is the vapor pressure of the liquid. If you now open the vessel, the atmosphere in the vessel is exchanged. This disturbs the equilibrium between the vapor phase and the liquid, so that more liquid evaporates. This process is called evaporation.

The vapor pressure is strongly dependent on the temperature. As long as the vapor pressure is lower than the ambient pressure, the liquid only evaporates on the surface. If the liquid is heated so much that the vapor pressure is identical to the ambient pressure, gas development takes place in the entire liquid phase and the liquid boils. The temperature at which the liquid begins to boil is called the boiling point. The energy required for evaporation is called the heat of evaporation or enthalpy of evaporation. The transition of a substance in the opposite direction to evaporation, i.e. from the gaseous to the liquid state of aggregation, is called condensation.

Boiling process


During the transition, the temperature remains constant, provided that the pressure also remains constant. All heat supplied is invested in the change in state.

In multi-phase systems, for example with the components water and acetone, the composition of the liquid and gas phase is usually different, the higher-boiling component accumulates in the liquid phase. Therefore, the boiling point rises as evaporation proceeds. The boiling process is best described by a boiling diagram.

The components are present in the liquid and the gaseous phase in different concentrations. This fact is used in thermal separation processes.

Example water

Under normal conditions, i.e. at an air pressure of 1013 hPa and a temperature of 20 ° C, water is liquid. If water is heated from below by a heat source in a vessel that is open at the top, a temperature gradient builds up. It is hottest on the ground, the surface temperature is still the same as the air temperature. Since warm water has a lower density than cold water, it rises, but colder water sinks downwards (convection). If heated slowly, all of the water can evaporate without bubbles rising.

If the temperature of the soil is higher than the boiling point of the boiling point at the prevailing hydrostatic pressure, the water evaporates. First, bubbles form on small bumps in the floor (evaporation nuclei), which rise upwards and allow cooler water to flow downwards, the water begins to boil. The bubbles contain water vapor or other gases that were dissolved in the cold water.

As long as bubbles rise and an intensive heat transfer through the wall is guaranteed, it is called nucleate boiling. If, on the other hand, a coherent vapor layer is formed, this is called Film boiling.

When transitioning from Nucleate boiling When the film is boiled, the heat transfer between soil and water deteriorates suddenly. With nucleate boiling the water is stirred through by the rising bubbles, this is missing with film boiling. This effect must always be taken into account when cooling hot objects (such as fuel assemblies) with water.

When still boiling, the boiling temperature has not yet been reached in the entire vessel. The bubbles condense and collapse in the colder water. The upper layers of the water are additionally heated by the rising hot steam bubbles until the entire body of water is evenly heated.

The warming of the surface water also leads to a warming of the vapor phase above. If the surface temperature of the water is greater than the boiling point at the prevailing air pressure (for example greater than 100 ° C at 1013 hPa), all of the water evaporates, provided that the supply of heat is not interrupted.

Delayed boiling

Main article: Delayed boiling

Water, hydrogen peroxide or alkalis (e.g. caustic soda) without dust particles or gas bubbles can also be heated above the boiling point in clean vessels without boiling. The smallest disturbances, such as vibrations that result in thorough mixing, can lead to an explosive separation of the liquid from the vapor phase, which is known as Delayed boiling designated. Because of this, solutions are added in chemistry that are at risk of delayed boiling, Boiling stones made of clay or pumice stone, which are not attacked by the chemical, but thanks to their porous structure facilitate the formation of small bubbles so that there is no delay in boiling.

See also

Category: Thermodynamic Process