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.
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