Page 20 - Curriculum Visions Dynamic Book

Boiling
When water boils, the water gets enough heat energy for molecules to push apart inside the water as well as on the surface. That is why bubbles form inside the liquid. The bubbles do not contain air – they contain only water vapour. We recognise bubbles forming inside the water as a sign of boiling. Boiling is thus a very fast form of evaporation.
(Left, right, and below) Boiling is the change from liquid to
gas within the water as well
as at the surface. That is what causes it to bubble. Each bubble contains water vapour (not air).
Many of the particles can now escape from the liquid surface.
Clumps of particles move in different directions, leaving almost empty spaces containing only fast-moving particles – we see these gaps as bubbles in boiling water.
Water vapour occupies much more space than liquid water. So, as liquid turns to vapour, bubbles swell. They are also less dense than the liquid, and so they rise through the liquid.
The higher the bubbles rise, the less weight of water lies above them, so the less pressure there
is on the vapour. As a result, the vapour expands, and the bubbles grow as they approach the surface. When they get to the surface, the bubbles then burst, sending water vapour (not air) into the air above.
It is possible to keep water very tightly confined. If this is done, the water cannot boil, and so it becomes superheated, and its temperature rises above normal boiling point. When the pressure is released, steam is produced very violently. It is also possible to produce steam and keep it confined so that it cannot expand and then allow it to expand. Again, the change in volume occurs very quickly.
Temperature
Boiling starts Time
Boiling ends
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