29 December 2010

Burst Pipes

It has been a very hard winter in norn iron, or as the BBC has it, Northern Ireland. Temperatures sank to - 18.7°C — about 0°F — and stayed below zero for weeks.
It’s thawing now, and the BBC News and Radio 4’s Today programme constantly tell us that the pipes are bursting because of the thaw. There are major problems, not only with burst pipes in private houses and businesses, but also with the mains. There is now a drought in many areas, with the mains shut off. People are advised to go to various depots to collect bottled water, or water from a tanker. Some people have had no water from before Christmas, and this misery is expected to continue for several days.
Well, the effects of burst pipes are certainly due to the thaw. But pipes do not burst because of the thaw.
This should be obvious to anyone who has ever had a drink with ice in it; the ice floats. And not because you can see the air bubbles — posh bars will serve your gin and tonic with ice made from degassed water, yet the ice still floats.
If you make ice cubes, the amount or mass of ice must be the same as the mass of water you put in. Ice floats because its density is less than that of water. And it’s density is less…because…it must be bigger.
Most substances expand as the get warmer, and contract as they cool. Water is an exception, and you will vaguely recall the ‘anomalous behaviour of water at 4°C’. (Or if you want to be pedantic, 3.95°C). 
Above 4°C, water expands as it gets warmer, and below 4°C it also expands. This doesn’t matter much, unless the water freezes. The lower the temperature, the more the ice expands, and the power of this expansion is impressive, enough to burst pipes. And, if you didn’t put anti-freeze in your car’s radiator, the block and radiator will burst. 
So, pipes burst during the freeze, but as the water is frozen solid as ice, there is nothing to see. Only when the ice melts, are the bursts evident, when the water can flow again.
So, BBC, it’s not the thaw that causes the bursts, it just makes them evident.

PS. The Guardian web site is just as bad: Large numbers of homes in Northern Ireland are likely to be without water for days to come, the national provider said, after a thaw in icy conditions caused pipes to burst.