
In a liquid, the particles are
fairly close together. They are free to
change places. There are
some forces between the particles. These forces keep most of the particles
together in the liquid. As the liquid is heated, the particles move
faster and faster. Eventually they move
so fast that they
break free from each other. When this happens, we say the liquid has
boiled. The
vapour that is formed is made from the liquid particles flying around at
high speeds in
all directions. This is shown in the second diagram. The particles are the
same as they were in the liquid, they are no bigger or smaller. It is
what they are doing that
changes.