Hot ice

Hand warmers containing a liquid in a flexible plastic shell with a small, curved metal disc are commonplace. The warmer is activated by inverting the disc, which presents nuclei to support the formation of crystals from a supersaturated solution. Needles propagate out from the nucleation site and within a matter of seconds the contents have solidified. The process is exothermic, generating heat.

This effect can be reproduced in the lab to offer more control over the process and produce attractive stalagmite formations.¹

I use this demonstration when developing students' ideas of particle models, particularly with respect to heating and cooling curves and changes of state. While the demonstration is technically crystallisation from a supersaturated solution, it is useful as a model for freezing. There are many videos on YouTube showing the freezing of supercooled water but it is not trivial to produce a sample on demand for use in front of a class,²  so this demonstration provides a suitable alternative. 

With an accompanying video, Declan Fleming presents how to set up and teach a 'hot ice' experiment.