Wikipedia
Ammonium persulfate
Ammonium persulfate (APS) is the inorganic compound with the formula (NH)SO. It is a colourless (white) salt that is highly soluble in water, much more so than the related potassium salt. It is a strong oxidizing agent that is used in polymer chemistry, as an etchant, and as a cleaning and bleaching agent.
The dissolution of the salt in water is an endothermic process.