Evaporation facts for kids

Kids Encyclopedia Facts
Evaporation
A simple picture explaining the evaporation of water, though in real life you can not see the water, but only steam
Watervapor cup
Aerosol of microscopic water droplets suspended in the air above a hot tea cup after that water vapor has sufficiently cooled and condensed. Water vapor is an invisible gas, but the clouds of condensed water droplets refract and disperse the sun light and so are visible.
Water vapor pressure graph
Vapor pressure of water vs. temperature. 760 Torr = 1 atm.

Evaporation is when a liquid becomes a gas without forming bubbles inside the liquid volume. If bubbles are formed we are talking instead about "boiling".

For example, water left in a bowl will slowly disappear. The water evaporates into water vapor, the gas phase of water. The water vapor mixes with the air.

The reverse of evaporation is condensation.

When the molecules in a liquid are heated, they move faster. This makes them full of energy and so the particles collide with each other, and eventually they become so far apart that they become a gas.

Differences between evaporation and boiling

During evaporation only the molecules near the liquid surface are changing from liquid to vapor. During boiling the molecules inside the volume of the liquid are also changing to vapour. For this reason during evaporation no bubbles are formed, instead they are formed during boiling.

Evaporation can happen at any temperature, while boiling happens only at a specified temperature called the "boiling point". Evaporation happens slowly, but boiling happens quickly.

Rate of evaporation

Some liquids evaporate more quickly than others. There are many factors that affect the evaporation rate.

The rate of evaporation depends on the liquid's exposed surface area (faster when increased), the humidity of surroundings (slower when increased), the presence of wind (faster when increased) and the temperature (faster when increased).

Liquid with high boiling points (those that boil at very high temperatures) tend to evaporate more slowly than those with lower boiling temperatures.

Evaporation is a very essential part of the water cycle.

Thermodynamics

Evaporation is an endothermic process, in that heat is absorbed during evaporation.

Applications

  • Industrial applications include many printing and coating processes; recovering salts from solutions; and drying a variety of materials such as lumber, paper, cloth and chemicals.
  • The use of evaporation to dry or concentrate samples is a common preparatory step for many laboratory analyses such as spectroscopy and chromatography. Systems used for this purpose include rotary evaporators and centrifugal evaporators.
  • When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low humidity, heat (from the sun), and wind. In a clothes dryer, hot air is blown through the clothes, allowing water to evaporate very rapidly.
  • The Matki/Matka, a traditional Indian porous clay container used for storing and cooling water and other liquids.
  • The botijo, a traditional Spanish porous clay container designed to cool the contained water by evaporation.
  • Evaporative coolers, which can significantly cool a building by simply blowing dry air over a filter saturated with water.

Combustion vaporization

Fuel droplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat (energy) can also be received by radiation from any hot refractory wall of the combustion chamber.

Pre-combustion vaporization

Internal combustion engines rely upon the vaporization of the fuel in the cylinders to form a fuel/air mixture in order to burn well. The chemically correct air/fuel mixture for total burning of gasoline has been determined to be 15 parts air to one part gasoline or 15/1 by weight. Changing this to a volume ratio yields 8000 parts air to one part gasoline or 8,000/1 by volume.

Related pages


Evaporation Facts for Kids. Kiddle Encyclopedia.