A burner is placed below a beaker containing water. A molecular depiction of the water in the beaker is also shown. Space-filling models representing water molecules are depicted wherein 1 large sphere represents 1 oxygen atom and 2 small spheres represent hydrogen atoms in each molecule. Water molecules are connected to the adjacent water molecules through hydrogen bonds represented by dotted lines. The water molecules vibrate slightly.
Boiling occurs when all the water molecules have so much energy that their motions break most of the hydrogen bonds. Then the liquid rapidly changes to gas bubbles (steam). If you keep adding heat to boiling water, the temperature stays the same because all the heat is used to break hydrogen bonds. The free molecules (steam) carry away energy. Click the burner for a detailed look.