It is a blue-green crystalline solid when hydrated and white when anhydrous. It is a reducing agent. It reacts with oxygen in the air to produce a brown coating of iron(III) sulfate and iron(III) oxide. This coating is very corrosive. It dissolves in water to make a blue-green solution. When it is heated strongly, it breaks down into sulfur dioxide, sulfur trioxide, and iron(III) oxide. It reacts with nitric acid to make nitric oxide. It also reacts with chlorine to make iron(III) sulfate and iron(III) chloride.