Why does water expand when frozen?

Unlike most other substances, water expands when it freezes. This is due to its unique molecular structure, where one oxygen atom bonds with two hydrogen atoms.

The larger oxygen atom moves to one side of the molecule while the hydrogen atoms move to the opposite side, giving water molecules a distinctive, bent structure. When water freezes, the molecules become more ordered and arrange themselves in a hexagonal, crystal configuration that increases the space between the molecules.

"Because water has this funny bent structure, those atoms pack most efficiently in a crystal structure," says physics professor Stephen Morris of the University of Toronto.

What happens to water pressure when it freezes?

And as water expands, the pressure it puts on a container — a water pipe, for example — increases dramatically.

The increased pressure in the pipe is "enormous," says Morris. "It's thousands of times of higher pressure."

How do you stop water from freezing in pipes?

"You can keep water from freezing if you put enough pressure on it," said Morris. "But no pipe's going to give you that amount of pressure."

There are ways to make it harder for water to freeze, however.

Flowing water is harder to freeze than stagnant water. To prevent bursting pipes, it's best to keep water running, says Morris. The water in a main is generally warmer than in a pipe, which helps keep it liquid, he says.

Keeping pipes well-insulated can also help, he says.