Re: Why does ice sometimes stick to your skin?

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.