MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY MARTYN F. CHILLMAID / SCIENCE PHOTO LIBRARY
Chemical hand warmer. The pad contains a supersaturated sodium acetate solution. Super saturation occurs when a chemical is present in a solution in high concentrations, normally sufficient to crystallise out of solution. However, this chemical can remain supersaturated at room temperature until it is activated by pressing the small metal disk (lower left). This prompts nucleation and crystallisation, releasing heat. Unlike other types of heating pad, this one can be heated and reused, as the process is reversible.
Model release not required. Property release not required.