Please watch animation Phase Changes on
the book web site.
As
heat is added to a solid material, the particles of the solid begin to
vibrate
faster. At a certain temperature, they begin to slide past each other
and the
material melts. At this point the
energy added goes in to breaking the attractive forces so while melting
is
occurring, the temperature does not change.
For water it remains 0 degrees C. If the material is heated
further, some
of the particles obtain enough kinetic energy to break their bonds to
other
particles and they enter the vapor phase. As more heat is added,
particles enter
the gas state at a faster rate. When the pressure of the surrounding
air is
equal to the pressure of the vapor, boiling begins and vapor bubbles
can form
inside the material instead of just at the surface. At the boiling
point, heat
energy added to the material is used to break the remaining
intermolecular bonds
and create gas vapor, so boiling occurs at a constant temperature.