|MadSci Network: Earth Sciences|
An interesting question. To answer, maybe we should start by considering why ice sticks to anything. And to answer that, we need to start with how is ice formed in the first place. On a big scale, ice is easy. You just take some water and make it very cold. But at the molecular scale it is much more fascinating. Ice is made from water and water, as you know, is "H2O" - that is, it is composed of two hydrogen atoms attached to a single oxygen atom. These atoms are arranged in a "V-shape" with the hydrogens at the tips of the "V" and the oxygen at the point. What you may not know, though, is that oxygen also has two extra pairs of electrons - two "lone pairs" - that dangle from it so that the water molecule is actually a "tetrahedron". A tetrahedron is a four side solid figure, with four points. (A tetrapak for cream is an example.) Water molecules interact when one of the lone pairs on an oxygen reaches out for the hydrogen atom on an adjacent water molecule. This is called "hydrogen bonding" and it is why water is a liquid at room temperature and not a gas. All of the water molecules are connected with their neighbours through hydrogen bonding interactions. In liquid water, although every water is connected, each molecule of water can break its old connections and make new ones. The result is that water flows. Each water molecule gets passed hand-to-hand from one molecule of water to the next. In ice, the temperature is too cold for the water molecules to break any connections and so the water molecules are stuck in place. A rigid three dimensional network of hydrogen bonded water molecules is formed - with none of the water molecules changing place. So, what has this to do with ice sticking? Well, if you think about the surface of the ice, there are going to be some water molecules that line the surface with their oxygen atoms facing outwards. These oxygens will not be able to "hydrogen bond" with any other water molecules because they are at the surface and there are no more water molecules to bond with! And some water molecules will be at the surface with their hydrogen atoms pointing out. They will also be looking for a partner to bond with but will be left unsatisfied. The result is that at a molecular level there are an awful lot of molecules that are left "hanging" - left searching for a partner to join with in a hydrogen bonding arrangement. When ice comes in contact with another surface that is capable of forming hydrogen bonds, then it will stick to that surface. Your skin has a fairly high moisture content and is therefore very suitable for forming these interactions - so, sometimes ice sticks to your skin. But if ice has all of these grabby molecules on its surface, how come it is slippery? Because above a certain temperature, the surface of the ice becomes mobile. The water molecules are able to move around on the surface, hopping from water to water and skipping along. The temperature may be below 32F (0 C) but the last molecules on the surface behave as if they are liquid water and each of these effective water molecule acts like a molecular ball bearing making the surface slippery. This is why ice doesn't always stick to your skin. Sometimes it just slides past. But chances are, if you have moist skin, then the water molecules on your skin will grab hold of the water molecules in the ice and hang on good and tight. Hope this answers your question. Good luck with your computer class.
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