Water comes in three different forms, or phases: liquid, solid or gas. Trouble tends to arise right at the point where one phase changes to another. Freezing, turning liquid water into ice, comes with expansion. Ice occupies 9 percent more volume than the equivalent amount of liquid water. This can do serious damage to home plumbing. Copper pipes are vulnerable to freezing temperatures, but PVC tends to burst, too, when ice forms in it. Outside faucets are the most exposed.