Normally you have to add heat to a solid to make it melt, but now a French group has made an astounding discovery: it has found the first example of a substance that is liquid at low temperatures, but which freezes when heated! Hans-Peter Trommsdorff of Joseph Fourier University in Grenoble and his colleagues mixed alpha-cyclodextrine - which is basically rings of six glucose molecules - with 4-methylpyridine and water to make the strange substance.

At room temperature, the mixture is a clear liquid, but on being heated to temperatures between 45 and 75 °C it turns into a white solid. It is important to note that this is not some sort of gelling, but a genuine phase transition to a solid.

The underlying physics is based on hydrogen bonds: some that would normally help to hold the cyclodextrine together are broken at modest temperatures. This allows new hydrogen bonds to form, making a solid. At higher temperatures, around 95 °C, the bizarre material reliquifies.

Further reading

M Plazanet et al. 2004 J. Chem. Phys. 121 5031.

Compiled by Steve Reucroft and John Swain, Northeastern University