Properties and reactions
Arsenic trioxide is an amphoteric oxide, and its aqueous solutions are weakly acidic. Thus, it dissolves readily in alkaline solutions to give arsenites. It is less soluble in acids, although it will dissolve in hydrochloric acid.With anhydrous HF and HCl, it gives AsF3 and the trichloride:
As2O3 + 6 HX → 2 AsX3 + 3 H2O (X = F, Cl)Only with strong oxidizing agents such as ozone, hydrogen peroxide, and nitric acid does it yield arsenic pentoxide, As2O5 or its corresponding acid:
2 HNO3 + As2O3 + 2 H2O → 2 H3AsO4 + N2O3In terms of its resistance to oxidation, arsenic trioxide differs from phosphorus trioxide, which readily combusts to phosphorus pentoxide. Reduction gives elemental arsenic or arsine (AsH3) depending on conditions:
As2O3 + 6 Zn + 12 HNO3 → 2 AsH3 + 6 Zn(NO3)2 + 3 H2OThis reaction is used in the Marsh test. Structure
In the liquid and gas phase below 800 °C, arsenic trioxide has the formula As4O6 and is isostructural with P4O6. Above 800 °C As4O6 significantly dissociates into molecular As2O3, which adopts the same structure as N2O3. Three forms (polymorphs) are known in the solid state:  a high temperature ( > 110 °C) cubic As4O6, containing molecular As4O6, and two related polymeric forms. The polymers, which both crystallize as monoclinic crystals, feature sheets of pyramidal AsO3 units that share O atoms. Society and culture
Environmental effects
Smelting and related ore processing often generate arsenic trioxide, which poses a risk to the environment.
Elimination is rapid at first (half-life of 1–2 days), by methylation to monomethylarsonic acid and dimethylarsinic acid, and excretion in the urine, but a certain amount (30–40% in the case of repeated exposure) is incorporated into the bones, the muscles, the skin, the hair and 
the nails (all tissues rich in keratin) and eliminated over a period of weeks or months. The first symptoms of acute arsenic poisoning by ingestion are digestive problems: vomiting, abdominal pains, diarrhea often accompanied by bleeding. Sub-lethal doses can lead to convulsions, cardiovascular problems, inflammation of the liver and kidneys and abnormalities in the coagulation of the blood. These are followed by the appearance of characteristic white lines (Mees lines) on the nails and by hair loss. Lower doses lead to liver and kidney problems and to changes in the pigmentation of the skin. Even dilute solutions of arsenic trioxide are dangerous on contact with the eyes. The poisonous properties are well known and the subject of an extensive literature.Chronic arsenic poisoning is known as arsenicosis. This disorder affects workers in smelters, in populations whose drinking water contains high levels of arsenic (0.3–0.4 ppm), and in patients treated for long periods with arsenic-based pharmaceuticals. Similarly, studies on workers exposed in copper foundries in the U.S., Japan and Sweden indicate a risk of lung cancer 6–10 times higher for the most exposed workers compared with the general population. Long-term ingestion of arsenic trioxide either in drinking water or as a medical treatment can lead to skin cancer.