Arsenous acid, also known as arsenious acid, is a chemical compound with the chemical formula H3AsO3. It is a weak acid and one of the oxoacids of arsenic. Arsenous acid exists as a white or colorless crystalline solid that is sparingly soluble in water.

Arsenous acid is formed when arsenic trioxide (As2O3) dissolves in water. It is important to note that arsenous acid can undergo further reactions in water to form other arsenic species, including arsenic acid (H3AsO4) and arsenite ions (AsO3^3-).

Some key properties and uses of arsenous acid include:

1. Preservative: Historically, arsenous acid was used as a preservative in the preservation of wood and animal specimens. However, its use has been largely abandoned due to its high toxicity and potential health risks.
2. Chemical Reagent: Arsenous acid is used as a chemical reagent in certain chemical reactions and laboratory studies.
3. Insecticide and Pesticide: In the past, arsenous acid and its derivatives were used as insecticides and pesticides. However, due to their extreme toxicity and potential environmental hazards, their use as pesticides has been restricted or banned in many countries.
4. Medicinal Use: Arsenous acid and its compounds were used in traditional medicine for various purposes. However, their use in medicine has been largely discontinued due to their toxic effects and the availability of safer alternatives.

Arsenous acid is highly toxic and poses serious health risks if ingested, inhaled, or absorbed through the skin. It should be handled with extreme care, and unnecessary exposure should be avoided. Due to its toxicity, the use of arsenous acid and its compounds has been restricted in many applications, and safer alternatives are now used for most purposes.