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Evaporation
Evaporation is the gradual change of a liquid into a gas at its surface. It can happen far below the boiling point.
What is evaporation?
Evaporation is the gradual change of a liquid into a gas at its surface. It does not require the liquid to reach its boiling point. A puddle can disappear at room temperature, wet clothes can dry on a cool day, and water left in an open glass slowly decreases even though none of it boils.
Inside a liquid, molecules are constantly moving and do not all have exactly the same kinetic energy. At the surface, some molecules have enough energy to escape the attractions of their neighbours and enter the air as vapour. At the same time, vapour molecules can return to the liquid. What we observe depends on the balance between these two processes.
The lower particles represent molecules in the liquid. A few leave the surface and enter the gas phase. The animation is conceptual: distances, speeds and trajectories are not to scale.
Evaporation and boiling are not the same
Evaporation occurs at the surface and can take place over a wide range of temperatures. Boiling occurs throughout the liquid when vapour bubbles can form and grow inside it. For pure water at standard atmospheric pressure, 100 °C is the familiar boiling-point reference, but evaporation happens far below 100 °C.
Surface process. It can happen slowly at room temperature and does not require bubbles.
Vapour bubbles form throughout the liquid when the vapour pressure is sufficient relative to the surrounding pressure.
Why does temperature usually make evaporation faster?
Raising the temperature shifts the distribution of molecular kinetic energies. A larger fraction of surface molecules can then overcome the intermolecular attractions holding them in the liquid. Other conditions being equal, warm water therefore tends to evaporate faster than cold water.
This does not mean that evaporation starts at one particular temperature. It is a continuous surface process. Temperature changes its rate; it is not an on/off switch.
Humidity, airflow and exposed surface
Temperature is only one control. When the air just above the liquid already contains a great deal of water vapour, the net loss of molecules from the liquid is reduced. Drier air generally favours faster net evaporation. Moving air can also remove humid air from above the surface and replace it with less saturated air.
Surface area matters for a simple reason: more exposed surface gives more molecules access to the liquid–air boundary at the same time. The same mass of water spread across a wide dish will generally evaporate faster than when it sits in a narrow glass under otherwise similar conditions.
Put the same measured amount of water in a narrow glass and a wide shallow dish. Place them side by side so that temperature and airflow are as similar as possible. Comparing the remaining mass after several hours isolates the effect of exposed surface much better than putting one container in the sun and the other in the shade.
Why can evaporation cool a surface?
Molecules that escape tend to come from the higher-energy part of the molecular population. Their departure removes energy from the liquid that remains. Unless energy is supplied fast enough from the surroundings, the liquid or wet surface cools. This is evaporative cooling.
Sweating uses the same principle. Sweat is not useful merely because it makes the skin wet: when water evaporates from the skin, energy is transferred away. Airflow can strengthen the cooling sensation because it helps evaporation continue.
Where does the water go?
It does not disappear. The molecules are still H₂O, now dispersed in the air as water vapour. If conditions later favour the reverse process, they can return to the liquid phase by condensation. Evaporation and condensation therefore operate continuously together in the water cycle.
Why do clothes dry?
After spinning or wringing, water remains between and on the textile fibres. Molecules continually leave these wet surfaces. Dry air, warmth, a large exposed fabric area and good ventilation usually speed the process. That is why spreading a garment out is more effective than leaving it crumpled in a damp pile.
In a humid, poorly ventilated room, drying can be much slower even at the same temperature. The example shows why real evaporation is controlled by several variables at once.
Is the white cloud above a kettle water vapour?
Not exactly. Water vapour itself is invisible. The visible white plume contains tiny liquid droplets produced when hot, humid air cools and some of the vapour condenses. Very close to a kettle spout there can be a short transparent region before the visible droplets appear.
Do all liquids evaporate at the same rate?
No. Molecular interactions and vapour pressure matter as well as temperature, surface area and airflow. A more volatile liquid has a greater tendency to enter the gas phase under the same conditions. Ethanol, for example, is generally more volatile than water at ordinary temperatures.
Quick checks
No. It can evaporate at ambient temperature.
It increases exposed wet surface and improves contact with moving air.
The visible cloud is mainly tiny liquid droplets; gaseous water vapour is invisible.
It replaces humid air close to the wet surface and can sustain net evaporation.
What evaporation helps us understand
Evaporation connects microscopic molecular motion to phenomena we can see and measure: drying, cooling, humidity, cloud formation, weather and the global movement of water. It is also a useful lesson in experimental reasoning: if we want to test one factor such as surface area, the other major variables should be kept as constant as possible.