This iconic picture shows a minister gliding serenely across the ice, but how is thos possib;e. Why is ice slippery enouhgh?
At school we were taught the reason is the pressure of the skates melts the ice and the resulting water act as a lubricant. This is a wonderful explanation, put forward by James Thomson in 1849 based on the fact that pressure lowers the melting point of ice, and it seemed so satisfying that everybody took it for granted that it was correct and it was long taken as received wisdom.
But it can't be quite right; even if the pressure is sufficient to melt the ice when we are talking about ice skating, it is nothing like enough to explain how skis glide along (or why I fall over on icy pavements for that matter!)
So rival theories have been advanced. One is that friction could melt the ice. It's possible this could have an effect but we'd expect melt water from friction to trail behind the sliding object and so not provide much lubrication going forward.
A third idea, based on an observation by Michael Faraday, is premelting. According to this theory the water molecules in the interior of the ice are tightly locked into a crystal lattice that restrict their movement but ones on or close to the surface are bonded to fewer close neighbours, allowing them more mobility, so they can act rather like water. Computer simulations support this as a plausible reason why the surface of ice should act as though it is wet.
It's conceivable all three of the mechanisms outlined above play a part. But very recently a fourth idea has emerged: the skate or ski (or the sole of my shoe) disrupts the crystal lattice of the ice and creates a layer of disorder molecule that are not fixed in a lattice and yet not quite liquid water, but which can act a lubricant. So far this has only been investigated in computer simulations and at present other researchers are not sure how it should be interpreted.
So we have not finally answered the question 'Why is ice slippery?' but this new research does represent an important advance, and we have edged closer to understanding exactly what is happening.
To read more about this see Quanta Magazine here.
For a good video about the new research see Anton Petrov, 'For 200 Years We Were Wrong About Why Water Ice Is Slippery',
Picture by Sir Henry Raeburn - Reverend Robert Walker (1755 - 1808) Skating on Duddingston Loch. Public Domain.