Via a post on Panda’s Thumb, here is an New York Times article about the slipperiness of ice.
Water has always fascinated chemists, because it has some rather unusual properties. For one, you would expect that tiny water molecules would exist as gas even at temperatures well below room temperature – since it’s usually true that the larger the molecule is, the higher its boiling point. Also unlike most other substances, water ice floats on top of liquid water. In just about any other substance, the opposite is true – the solid is more dense than and sinks in the liquid.
Water is so unusual that some early chemists even considered it as a proof of the existence of a god. Here’s JL Comstock, writing in his 1845 book Elements of Chemistry:
The effects of temperature upon liquid water is distinguished by a peculiarity of a very striking kind, and exhibits a departure from the general laws of nature, for a purpose so wise and beneficient, as to afford one of the strongest and most impressive of those endless proofs of design and onniscience in the frame of creation, which it is the most exalted pleasure of the chemist, no less than of the naturalist, to trace and admire.
The New York Times article talks about the various explanations offered for why ice is slippery.
- Ice’s solid phase is less dense than its liquid, and a large amount of pressure can lower the melting point of ice.
- The friction of dragging something (like your shoes) across ice generates enough heat to melt the ice.
- Solid ice is coated with a very thin layer of liquid water, even at temperatures well below 0C (32F).
Of the three explanations for ice being slippery, the first one appears in the most books. Ironically enough, it appears to be the least significant effect – except at temperatures close to the freezing point. We don’t put very much pressure on ice by walking on it. The real answer may lie in a cmbination of the second and third explanations – or somewhere else entirely.
It is, after all, a slippery subject.
Also interesting is that there is more than one kind of ice:
At higher pressures, the usual hexagonal structure breaks down, and the bonds rearrange themselves in more compact, denser crystal structures, neatly labeled with Roman numerals: Ice II, Ice III, Ice IV and so on. Scientists have also discovered several forms of ice in which the water molecules are arranged randomly, as in glass.
(What, no ice nine?)
These other arrangements of water molecules are more dense than the form of water ice that we observe in our freezers. That’s interesting too, but one ice researcher points out that these many different forms of ice can help us better inderstand how water molecules interact with other substances. This is pretty important for creatures like us, who are made of mostly water!