Watch this science video tutorial from Nurd Rage on how to make hot ice with Dr. Lithium. This is the complete guide to making hot ice, more correctly called sodium acetate. See how to create it, fix it, and use it. All methods from baking soda and vinegar to laboratory synthesis are shown.
The basis for hot ice is sodium acetate trihydrate, which is heated above its melting point and then cooled below its melting point so it's supercooled. It's still liquid and quickly solidifies when a seed crystal is introduced.