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| Earth on Oct 18, 2009

What’s so special about water?

Water is unusual in that it can occur naturally in three distinct forms – gas, liquid and solid – at the same time.

Good old H20…you might say it’s both ordinary and extraordinary.

Water is ordinary in that there’s so much of it, both on and around our planet. The oceans alone contain around a hundred million billion tons of water. At any one time, there’s also an amazing amount of water in the air.

Water is unusual in that it can occur naturally in three distinct forms – gas, liquid and solid – at the same time. On a winter day, you can see solid water – ice atop a partly frozen creek – and liquid water flowing in the creek. And if you look up, you see evidence for water vapor, water’s gaseous form, in the clouds floating in the sky.

Most substances contract as they freeze, but water expands. If you’ve ever filled a container with water all the way to the top and put it in the freezer, you know this from experience. The expanded ice cracks the container or pushes off the top. Also, a bucket full of ice is lighter than the same sized bucket full of water. That’s why ice floats on water.

Water also is a substance with a high “heat capacity”, which means that it’s slow to change its temperature. This quality may make you impatient as you’re waiting for your kettle to boil, but it’s the same quality that allows Earth’s oceans to serve as controls in helping to prevent extremes in climate.