What is so special about water? It covers 70% of our planet in the form of the oceans and it's essential for life, but it is not just because of it's abundance or contribution to our biology that make it interesting. The chemistry of water, or H2O, a relatively simple looking molecule, is not fully understood. Why does this matter?
Surprisingly, when usually pollution is the main focus, water is the most abundant greenhouse gas. It absorbs 3-4 times more radiation from the sun than CO2, and without it in the atmosphere the earth would be 33ºC cooler and frozen over. Oddly water absorbs more energy than it should, something called the 'water vapour continuum'; research is ongoing to explain this. If the ice caps keep melting there will be more water in the atmosphere, will this speed up global warming?
Another interesting phenomena about water is that when it freezes into a solid, it becomes less dense. Normally when molecules turn from a solid to a liquid the molecules get closer together, but water is different. This is due to differences in the structure between water and ice. Water follows a tetrahedral (triangle-faced pyramid) structure, whereas ice has a hexagonal structure with large holes in the centre of each hexagon. These large holes make the ice less dense.
That is why ice floats on top of water, if it didn't there would be no life on the planet. Imagine if ice in the ocean sank every time it froze, it would kill everything below it! However water doesn't freeze easily because of this phenomena, it needs something called a 'nucleation site', somewhere the freezing can begin. This is similar to the gas coming out of a fizzy drink; if you look at the glass the bubbles usually form in the same place at a nucleation site. This can be on a speck of dust or an imperfection in the glass.
Did you know that snowflakes are not frozen raindrops? They are formed from water vapour 'nucleating' on suspended dust particles in a cloud.
Demonstrators at the A molecule's eye view of water stall showed the crowds how filtered water (to remove dust and other particles) can be 'supercooled' to below 0ºC but won't freeze as it needs a 'nucleation site' for ice to form on, in this case a block of pre-made ice. When the supercooled water was poured onto an icecube, it froze on top instantly creating a spire of ice.
So why research this strange behaviour? Well using super-computers, it can be predicted how easily ice forms on different surfaces and this can be used to find a material that can prevent ice forming on aircraft wings.
So weird wonderful water, important for biological life, global warming, the safety of aircraft and other phenomena in the universe? With 15 different 'types' of ice in the universe (on earth we have Ice1) the researchers have a lot more work to do!
See the A molecule's eye view of water exhibit page for more images, videos and links, or download the 'Waterfall' game from iTunes showing how molecules of ice turn into water.