Why Do Water Pipes Sometimes Burst in Winter?
Cold water pipes may burst in winter when the outside temperature falls below 0°C (32 °F) and the water turns to ice. The pipe’s walls crack to relieve the pressure caused by the fact that ice requires nearly one-tenth more space than the water. One cubic foot of water makes 1.09 cubic feet of ice. To prevent a freeze up, a heat insulator is wrapped round the pipes.
Water has very unusual properties. Apart from expanding when frozen, it requires more heat to warm it than any other common substance. In other words it has a high specific heat. In nature there are some obvious advantages in these peculiarities. The expansion of ice causes the breaking up of clods of water-filled soil on cold winter nights to leave a fine tilth admirably suited to spring sowing.
Water’s high specific heat means that the sea takes longer to warm up than the land and longer to cool down. Thus the sea acts to prevent extreme changes in temperature between the seasons. We grew up with water all around us and so 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.
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. Water molecules are attracted to each others’ opposite sides. These potentials create weak bonds that are called Hydrogen Bonds. Hydrogen bonds, whilst not as strong as covalent bonds or ionic bonds, are stronger that van der Vaals forces.
This extra hydrogen-bond ‘glue’ holding water molecules together is the reason why water is a liquid at everyday temperatures and pressures (another one of the reasons life exists). Other chemical compounds similar to H2O, but without the benefit of Hydrogen bonds, are all gases in typical Earth temperature ranges.
The Hydrogen-bonds in water are also the reason why water has such a high specific heat capacity (the amount of energy required to raise the temperature of water one degree); this helps dampen our weather and stops the Earth changing in temperature too rapidly. It also means that water is great for carrying energy around (like in power stations and hydronic central heating systems). It’s also why it takes a lot of energy to boil a kettle to make a nice cup of tea!
As water cools, like other liquids, the molecules slow down and get denser. A competing force, however, is the desire for the water molecules to align with other water molecules based on their Hydrogen bonds, and this causes expansion. Below the temperature of 3.98°C down to 0°C, this alignment expansion process wins out against the desire of slower molecules to get closer, and density decreases.
Whilst not completely understood, it is this expansion that causes beautiful snowflakes to form with their characteristic six points. This decrease in density continues until the water finally freezes to form ice. Water expands to form ice which has a volume up to 9% greater than the water it came from. It is this reason that icebergs float (being less dense than the water they displace). Again, it seems natural to us that ice floats, because we grew up this all effect all around us, but this is atypical.
Most other liquids, when freezing, do not form a solid ‘crust’ on top. As they cool, the solid formed, being denser, drops to the bottom of the cooling liquid and the solid grows up from the bottom! This irregular behavior of water is yet another reason that life exists. When water freezes, it floats to the top, forming a skin which insulates the water below. When a river or lake freezes on top, life continues below in the liquid left underneath.