This demonstration can be used to demonstrate a sinmple physical change.
At a more advanced level, it can be used to show the anomalous density of ice – especially if done in conjunction with a lava lamp where the solid wax will sink rather than float.
The Density of Ice
As long as it is carried out as described, there are no significant risks associated with this experiment.
Photo by Mike Mozart on Flickr. Used under a Creative Commons 2.0 license