L5 - Liquids & Vapours

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Even on a cool day, rain puddles can vanish and wet clothes dry out. The water becomes an invisible gas (called water vapour) which drifts away in the air.
When a liquid below its boiling point changes into a gas, this is called evaporation. It happens because some particles in the liquid move faster than others.
The faster ones near the surface have enough energy to escape and form a gas.

When a liquid evaporates, faster particles escape from its surface to form a gas. However, unless the gas is removed, some of the particles will return to the liquid.

Increase the temperature Wet clothes dry faster on a warm day because more of the particles (water molecules) have enough energy to escape.
Blow air across the surface Wet clothes dry faster on a windy day because the moving air carries escaping water molecules away before many of them can return to the liquid.
Increase the surface area Water in a puddle dries out more quickly than water in a cup because more of its molecules are close to the surface.
Reduce the humidity* If air is very humid, this means that it already has a high water vapour content. In humid air, wet washing dries slowly because molecules in the vapour return to the liquid almost as fast as those in the liquid escape. In less humid air, wet washing dries more quickly.

Boiling is a very rapid form of evaporation.
When water boils, as in the photograph on the right, vapour bubbles form deep in the liquid. They expand, rise, burst, and release large amounts of vapour.
Even cold water has tiny vapour bubbles in it, but these are squashed by the pressure of the atmosphere. At 100 °C, the vapour pressure in the bubbles is strong enough to overcome atmospheric pressure, so the bubbles start to expand and boiling occurs.
At the top of Mount Everest, where atmospheric pressure is less, water would boil at only 70 °C.

Evaporation has a cooling effect. For example, if you wet your hands, the water on them starts to evaporate. As it evaporates, it takes thermal energy away from your skin. So your hands feel cold.
The kinetic theory explains the cooling effect like this. If faster particles escape from the liquid, slower ones are left behind, so the temperature of the liquid is less than before.

Refrigerators use the cooling effect of evaporation. In the refrigerator on the right, the process works like this:

In the pipes in the freezer compartment, a liquid called a refrigerant evaporates and takes thermal
energy from the food and air.
The vapour is drawn away by the pump,
which compresses it and turns it into a
liquid. This releases thermal energy, so
the liquid heats up.
The hot liquid is cooled as it passes
through the pipes at the back, and the
thermal energy is carried away by the air.

Overall, thermal energy is transferred from the things inside the fridge to the air outside.

Sweating also uses the cooling effect of evaporation. You start to sweat if your body temperature rises more than about 0.5 °C above normal. The sweat, which is mainly water, comes out of tiny pores in your skin. As it evaporates, it takes thermal energy from your body and cools you down.
On a humid ('close') day, sweat cannot evaporate so easily, so it is more difficult to stay cool and comfortable.

When a gas changes back into a liquid, this is called condensation.
For example, cold air can hold less water vapour than warm air.
so if humid air is suddenly cooled, some of the water vapour may condense.
It may become billions of tiny water droplets in the air - we see these as clouds, mist, or fog.
Or it may become condensation on windows or other surfaces.
If condensation freezes, the result is frost.

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