Phase change
As the states of matter change from solid to liquid to gas, respectively, their composition changes as well. For example, in a solid, the bonds are stronger than hydrogen bonds. That allows the solid substance to have a definite volume and shape. However, when the heat is added to the solid and it melts to become a liquid, the bonds are considerably weaker, and in water, are simply hydrogen bonds.  A liquid has a definite volume but not a definite shape, and it thus takes the shape of the container in which it is. When more heat is added, the liquid substance evaporates and becomes a gas, which has no bonds at all. Gas is simply a formless collection of particles that tends to expand in all directions at the same time in order to occupy its full container. If a gas is not confined, the space between the particles will continue to increase. A gas has neither a definite volume nor a definite shape.