sodium thio sulfate.
The crystal of sodium thio sulfate has enough water to completely dissolve the solute. Heat the crystal gently it will turn into a a liquid solution. Along the solution to cooly slowly it will stay a liquid solution.
Drop one crystal of the solid sodium thio sulfate into the liquid solution and it will immediately start to crystalize as it is super aturate. Doing this under a microscope or magnifying class is an awesome experience as the observe can watch the crystals form.
An added bonus is the heat released. The forming of a solid from a liquid in sodium thio sulfate is highly exothermic. It helps the students understand that when water goes from a liquid to a solid (ice) it is an exothermic reaction.
A liquid solution of sodium thio sulfate is a super saturated solution.
It has great teaching opportunities in the classroom.