Sodium acetate (NaC2H3O2) heated in water then cooled has the unusual property of crystallising into a solid when it is disturbed.
It can be poured out of a beaker as a liquid and, upon hitting a surface, becomes a solid that is hot to touch – hence its other name, hot ice.
Sodium acetate is a salt which dissolves in water. Heating – to around 100 degrees Celsius (212 degrees Fahrenheit) – then cooling a mixture of the two allows more sodium acetate to dissolve to form a supersaturated solution.
The solution exists in a metastable state, analogous to a ball perched at the top of a hill, where the slightest nudge will make it roll down.
The trigger can be pouring the solution out of the container, or adding a seed crystal, causing the dissolved sodium acetate to come out of the solution and return to a solid. In our analogy this is like the ball rolling down the hill until it reaches flat ground and a lower energy state.
Along the way, the solid sodium acetate absorbs three molecules of water, becoming sodium acetate trihydrate (NaC2H302*3H20). These water molecules are not chemically bonded to the sodium acetate, representing a physical change. The process is exothermic (ie it releases heat) and, as a result, it’s often used in hand warmers.