Why is heat called a form of energy?

Because heat can be converted from one form to another.

Heat is also does work, for example it sets water molecules into motion when boiling ( convection currency ).

since energy is the ability to do work, heat does work.

molecules are free to separate from each other and will take on the shape of the container. molecules move completely at random and high speeds. they vibrate in a fixed position and do not move around much. molecules expand to the edge of their container.