It is a colorless solid. It can absorb water to make an oily liquid. It is a weak acid. It reacts with bases to make hypophosphite salts such as sodium hypophosphite. It is a powerful reducing agent. It breaks down when heated into phosphine and phosphorous acid.
It is made by protonation of hypophosphites.
It can be used as a reducing agent. It is also used to make medicines. It can be used to make illegal drugs, which is why it is restricted. It is used to plate metal on objects without using electroplating.