We were just discussing about why liquids in general are used in glass thermometers. I was wondering why water isn't considered a proper liquid. Is it because of the way is expands differently to others? The fact that is boils at 100 Celcius and freezes at 0 Celcius, whereas others will contract in alternate ways?
If water is not a proper liquid, what is it that makes mercury a proper liquid?
I mean I know mercury is a metal, and so has fairly high temperature expansion. But I've never seen liquids referred to in this way before, as "proper" and not so. It must be purely in a thermometer context, because we all know that water is just about the most "proper" liquid there is!
Best Answer
Water is the people's most popular liquid but it is also one that differs from almost all other liquids in some almost qualitative ways.
The most important reason why water is considered an "improper liquid" is that its density is higher than the density of its solid phase, the ice. For "proper liquids", it's the other way around – the solid phase is denser. For water, the maximum density occurs for 4 °C. (That's why a water-based thermometer couldn't distinguish temperatures above from those below 4 °C, even if it were positive.)
Also, its heat capacity per mole is very high – and almost none of the heat is stored in the molecular motion. The heat capacity is unusually constant between 0 and 100 °C, with a shallow minimum at 35 °C. When pressure goes up, the coefficient of expansion goes up and the viscosity goes down – it is the opposite for other liquids.
In 1892, Röntgen found an amusing "explanation" of these facts. Water isn't just a liquid, a single substance, but "ice dissolved in proper pure liquid water". This is the main microscopic idea behind water an "improper liquid".