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Heating Curves

Take a look at the Flash Animation on the right.  Here's an explanation of each part of a heating curve, starting on the left.
  1. A solid can heat up.  As heat is added, the temperature goes up.
  2. As the solid turns into a liquid (melts aka fusion), you're adding heat, but the temperature stays the same.  The heat energy is being absorbed and is being used to break the bonds that hold the atoms/molecules in fixed position.
  3. A liquid can heat up.  As heat is added, the temperature goes up.
  4. As the liquid turns to a gas (evaporation), you're adding heat but the temperature stays the same.  The heat energy is being absorbed and is being used to break the intermolecular bonds that hold the molecules together.
  5. A gas can heat up.  As heat is added, the temperature goes up.
  6. NOT SHOWN: If the pressure and temperature both go above the critical point, your substance can turn into a supercritical fluid.
Source: This flash animation is from North Carolina School of Science and Mathematics's TIGER (Teachers' Instructional Graphics Educational Resource).