Phosphorous acid (H3PO3) in its crystalline form is a chemical compound that contains phosphorus, hydrogen, and oxygen. It is used in various applications, including as a reducing agent in chemical reactions, as a fungicide in agriculture, and as a flame retardant in the manufacture of plastics and textiles.

Phosphorous acid crystals are known for their reducing properties, making them useful in industries where reduction reactions are required. In agriculture, they are employed to control fungal diseases in crops. Additionally, these crystals are used in the production of flame-retardant materials, contributing to fire safety in a range of products. When handling phosphorous acid crystals, safety precautions must be taken, and protective gear should be worn, as they can be harmful if not used properly.