h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It is not mentioned but the triple point exists BUT the specification includes a temperature AND a pressure. The triple point of water @ 0.01 degrees C is defined at the standard pressure of one atmosphere (or 760mmHg).

[Incidentally this explains why Antartica is the driest place on Earth. At the ambient temperatures, water exists almost only as a solid with little water vapour pressure above it.]

Other substances follow the same pattern. For instance, carbon dioxide has only a double point at standard pressure, ie. it will exist only as solid (dry ice) and gas. To produce CO2 liquid, a much higher pressure is required.

Slight point - the triple point of water is at 0.01 degrees C (273.16 Kelvin), but the corresponding pressure is considerably less than a standard atmosphere. I think the triple point pressure is about 6 kPa - around 6% of an atmosphere (101.325 kPa).

You may have implied this already, but unlike most other substances, the melting point *decreases* with increasing pressure! This makes ice-skating possible, as the pressure applied by the skater on the blades is sufficient to melt the ice, making it easy to travel on it. I do agree that at Antartic temperatures, the water is most likely to exist as a solid with a little water vapour, as both solids and liquids have corresponding saturated vapour pressures, i.e. the pressure at which the solid/liquid and vapour at a certain temperature are at equilibrium. This can also be used to calculate the composition of water vapour in the Antartic air - maybe someone could find this out?

Mikeo.

This is not what makes ice skating possible. The pressure required to reduce the melting/freezing temperature of water by 1K is about 121 atmospheres (about 12250 kPa). Assuming that a skater weighs 100kg (big I know but it eases the calculations) and the skate is 1mm wide and 20cm long (possibly too narrow and/or short) then the pressure exerted is 4900kPa or 48 atmospheres. This not enough for pressure melting to be a factor unless the ice is already close to melting already. The reasons for ice skating working are not fully understood but it is not due to pressure melting.
There is a web page about this and other common misconceptions at [Broken link removed by Moderator]from which I got most of my base information, although I have been more generous in dimensions for my calculations.
Wick

The actual edge of a skate is a lot thinner than 1mm.

They're also curved, so the length of contact is also reduced.

Get out your physics books.

It's not merely pressure that melts the ice, making ice-skating smoother, but the friction from the movement of the blade. The ice under a skate will show little sign of melting without the friction!

Yet skaters still manage to fall over when stationary...

Yes, we do!