Antimony trioxide is a white solid. It is the most common antimony compound. It does not dissolve in water. It reacts with oxidizing agents to make antimony pentoxide and with reducing agents to make antimony or stibine. It reacts with concentrated acids to make antimony(III) salts and dissolves in strong bases.
Antimony trioxide is found in two minerals, valentinite and senarmontite. Valentinite is a white mineral that is sometimes pale yellow.
Antimony trioxide is made when antimony is made. Stibnite is heated with air to make antimony trioxide. It is separated from arsenic by the boiling of the arsenic trioxide before the antimony trioxide boils. Antimony trioxide can also be made by a two step process. Stibnite is burned in air with calcium chloride to make calcium sulfate and antimony trichloride, which is reacted with water to make antimony trioxide.
Antimony trioxide is mainly used as a flame retardant. It is also used to make glass, enamel, and ceramic opaque (not clear). Some pigments have antimony in them. It is also used as a catalyst for making plastics. It is also used as a catalyst for vulcanizing rubber, making it more "rubbery" and less likely to crack.
Antimony trioxide is somewhat carcinogenic. It is somewhat toxic when eaten or breathed in.