Why do water pipes burst, and why do they always fail the same way? These are two interesting questions.

Water pipes burst because the water inside them expands as it gets close to freezing, and this causes an increase in pressure inside the pipe. When the pressure gets too high for the pipe to contain, it ruptures.

This expansion phenomenon seems natural, but interestingly, it is a chemical anomaly. Most liquids do not expand just before transition to solid. You should be thankful for this; it is one of the reasons that life exists.

When a liquid cools, the molecules slow down (temperature really is just a measure of the average kinetic energy of the molecules). This slowing down allows the molecules to get closer together and increases the density of the liquid. This happens with water too, and when water is cooled down, it gets denser and denser, down to 3.98°C. Then, something interesting occurs; it starts to expand again.

You can see this effect on the graph below which shows how the density of pure water changes with temperature (at one atmosphere). You can see how density reaches a maximum at 3.98°C.

Density of Water Graph

Because of the shape of a water molecule, it is slightly polarized. The electrons buzzing around it are more likely to be on one side of the molecule than the other (called a dipole), and this asymmetry creates a slight potential. Read more here.

1From “Why Cold Weather Makes Your Water Pipes Burst”, Nick Berry, DataGenetics, December 11, 2013