Evaporate

Water evaporating on a hotplate

The evaporation is the phase transition of a liquid or a liquid mixture into the gaseous aggregate state .

For the evaporation of a liquid, the enthalpy of evaporation must be applied. Heat is extracted from the environment or the liquid . If no external heat is supplied to the system, a closed system reaches a certain state of equilibrium , which is described by the temperature and the phase proportions in the physical states involved.

Forms of evaporation

A distinction is made between two forms of vaporization

A liquid boils when the temperature-dependent saturation vapor pressure of the liquid is greater than the pressure of the superimposed gas phase.

During evaporation, a liquid is superimposed with another gas that is superheated (the state of the superimposed gas is above the critical point in the Ts diagram ). Depending on its temperature, the superimposed gas can absorb a certain volume or mole fraction of the liquid. The partial pressure of the evaporating liquid is established in equilibrium above a liquid, which corresponds to the saturated state. The ratio of the partial pressure of the vaporized phase to the partial pressure at saturation in the superimposed gas is referred to as the relative humidity φ.

The liquid evaporates as long as the saturation vapor pressure is not reached in the gas phase.

Example: Isobaric heating, evaporation and overheating of water, state progression in the Ts diagram

example

On earth, water evaporates at an external air pressure of 1.013  bar up to a temperature of 100 ° C. The rate of evaporation is determined by the temperature of the liquid and the degree of saturation of the superimposed air with water vapor . With strong air movement ( wind ), the air above a water container is exchanged for unsaturated air at a high rate; the rate of evaporation of the water increases. Furthermore, the rate of evaporation increases as the temperature rises, since at a higher temperature the air can absorb more water (the saturation partial pressure is higher).

Water boils at a temperature of 100 ° C and an external air pressure of 1.013 bar. As long as it is an open system with constant pressure, the water evaporates. The mass flow of the boiling liquid is determined by the heat input to the liquid.