Phosphorous acid is colorless or white crystals with a garlic odor, featuring strong hygroscopicity and deliquescence. It is a dibasic weak acid with high reducibility; it is slowly oxidized to phosphoric acid in air, and decomposes above 180 ℃ into phosphoric acid and highly toxic phosphine (PH₃).

1. Chemical reducing agent: Used in chemical reduction reactions, metal refining and dye intermediate synthesis.
2. Raw material for phosphite salts: For the production of potassium phosphite, sodium phosphite, etc., applied in high-efficiency fungicides and fertilizers.
3. Plastic additives: Served as whitening agents and stabilizers for nylon and polyester.
4. Pesticide intermediate: Raw material for synthesizing organophosphorus pesticides, water treatment agents and flame retardants.
5. Electronics industry: High-purity grade is used in the manufacturing of electronic chemicals and semiconductor materials.