Boiling

Boiling, is in contrast to the evaporation, a quick change in state from liquid to gas, at which the vapor pressure of a liquid reaches the ambient pressure. Can boil both pure substances and mixtures.

Boiling

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

For single-phase systems, such as water, the temperature remains constant until the liquid is completely evaporated. This is based on the highly effective evaporative cooling of thermally highly loaded objects.

For multi-phase systems, for example, the components of water and acetone, the composition of the liquid and the vapor phase is generally different from the higher-boiling component accumulates in the liquid phase. Therefore, the boiling point increases with increasing evaporation. The boiling process is best described by a Siedediagramm. In this case, the components are present in different concentrations in the liquid and the gaseous phase. This fact is exploited in thermal separation processes such as distillation.

As water

Under normal conditions, ie at an air pressure of 1013 hPa and a temperature of 20 ° C, water is liquid. If water is heated in an open-topped vessel from below by a heat source, a temperature gradient builds up. At the bottom it is at its hottest, the surface temperature corresponds to the still air temperature. Because warm water is less dense than cold water, it rises to, but colder water sinks to the bottom ( convection). At slow heating all the water can evaporate without bubbles rise.

Is the temperature of the soil is higher than the temperature of the boiling point at the prevailing hydrostatic pressure, as the water evaporates. First (evaporation nuclei ) are formed at small unevenness in the floor of bubbles, which rise to the top and let cooler water flow down: the water begins to boil. The bubbles contain water vapor or other gases that had been dissolved in cold water.

As long as bubbles rise and intensive heat transfer is ensured through the wall, one speaks of nucleate boiling. Forms, however, a continuous vapor layer, this is called film boiling.

During the transition from nucleate boiling to film boiling, the heat transfer between the soil and water deteriorates rapidly. When boiling the water is stirred by the rising bubbles, this is missing in the film boiling. This effect must be taken with water during the cooling of hot objects (such as fuel ).

When silent boiling, the boiling temperature is not reached in the entire vessel. The bubbles condense in the colder water and break down. Due to the rising hot vapor bubbles, the upper layers of the water is heated further until the whole body of water is heated evenly.

The heating of the surface water also leads to a heating of the vapor phase above it. Is the surface temperature of the water is greater than the boiling point at the prevailing atmospheric pressure ( for example, greater than 100 ° C at 1013 hPa), all of the water, so far as the heat supply is not interrupted evaporated.

Siedeverzug

Water, hydrogen peroxide or alkali (for example sodium hydroxide solution) with no dust particles or gas bubbles can be heated in a clean vessel and about the boiling point of addition, without causing boiling. Smallest disturbances, such as vibrations, which involve a mix by itself, can lead to an explosive separation of the liquid from the vapor phase, which is called boiling. To avoid this, you add boiling chips to solutions of clay or pumice, which are not affected by the solution, but facilitate the porous structure, the formation of small bubbles so that it does not come to boiling.

Physical explosion

The strong increase in volume by evaporation of a substance, when a lot of heat is supplied to suddenly lead to a physical explosion.