Page 19 - Curriculum Visions Dynamic Book

When a liquid evaporates, it does so without any visible sign. That is why, if you leave a saucer with water in it,
all the water will eventually disappear almost mysteriously.
As heat is taken away, as on a cool night, the opposite is true, with more
gas molecules losing heat and so slowing down than liquid molecules gaining heat and breaking away. The result is the coating of water we find on cold surfaces early in the morning. It is condensation.
In a pure liquid, such as pure water, the molecules of the gas are exactly the same as the molecules in the liquid. But in a solution (which is a mixture), this is not true, and the vapour contains a greater concentration of the molecules that find
it easiest to evaporate. When seawater is heated, the water molecules evaporate, but the salt remains behind because the water molecules evaporate far more easily than the salt.
(Right) The gaseous form of water is called water vapour. Evaporation provides enough energy for water to leave the liquid state and become gaseous. Steam is gaseous water at boiling point or above.
Steam is an invisible gas. The white “steam” we can see coming from a kettle as it boils
is, in fact, tiny droplets of water that have condensed as the steam moves into cold air. The water vapour has condensed into its liquid state, which is why it will wet nearby surfaces. But if you look carefully at a kettle boiling, you will see that just beyond the spout of
the kettle there is a zone with no droplets. This is pure steam (that is, water as an invisible gas).
Liquid again – water vapour condensing as water droplets that we can see.
Water vapour – gas;
we can’t see the vapour.
Water boiling inside the kettle
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