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Why is ice cold?

Why is ice cold?
Amelia Newey from Warwickshire (age 5-14)

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One Response

  1. It doesn’t have to be. The reason all the ice you (or me) are ever likely to encounter is cold is because we’re experiencing it on Earth, under the pressure of our atmosphere.

    What makes ice different from water is just the arrangement of the molecules within it. They’re still H20 (two hydrogen atom and an oxygen atom), but they are packed closely together and regularly, giving the outward appearance that we recognise as a solid.

    So whether water is in its ice form depends on the packing of the molecules. This is affected by two things: how hot the molecules are, and how hard they are being squished from outside. The more heat there is, the more energy the molecules have, the faster they move and the harder it is to get them to stay in a regular structure. But one way to persuade them to jiggle around on the spot in a regular formation is by applying pressure from outside. So to form ice, you need it either to be cold, or to be at high pressure.

    On Earth the usual pressure is 1 atmosphere, or 10,000 Pascals – that’s the same pressure as a car exerts if it’s balanced on an area a metre squared. You don’t feel it because you’ve never know anything different, but this is always there pushing down on you. At this pressure, ice forms at 0 degrees Celsius. You’d think that at lower pressures, it would form at lower temperatures. This is the case for most substance, but not water, because it is what we scientists call ‘weird’.

    This graph http://chimge.unil.ch/En/mat/images/im1mat27.1.gif
    shows the interrelation of temperature and pressure. The white area is where water would be ice – if you look down, you can see that the highest temperature you can get ice is about 20 degrees, room temperature, but that’s at half atmospheric pressure on Earth.

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