Sulfur Trioxide - SO3

What is Sulfur Trioxide?

Sulfur trioxide is a chemical compound which is represented by chemical formula SO3. Sulfur trioxide exists in a number of modifications that differ in molecular species and crystalline form. In its liquid for it is colourless and fumes in air at ambient conditions. It is a highly reactive substance and a strong oxidising agent and it also acts as a fire hazard. It is thermodynamically unstable compound with respect to selenium dioxide.

Other names – Sulfuric anhydride


Sulfur Trioxide


1.92 g/cm³

Molecular Weight/ Molar Mass

80.066 g/mol

Boiling Point

44.9 °C

Melting Point

16.9 °C

Chemical Formula


Sulfur Trioxide Structure – SO3

Sulfur Trioxide Structure

Physical Properties of Sulfur Trioxide – SO3


No odor


Colorless to white crystalline solid

Covalently-Bonded Unit




Hydrogen Bond Acceptor



Soluble in water.

Chemical Properties of Sulfur Trioxide – SO3

  • Sulfur trioxide reacts with water forming sulfuric acid. The chemical equation is given below.
  • SO3 + H2O → H2SO4

  • Sulfur trioxide reacts with a base sodium hydroxide forms sodium hydrogen phosphate. The chemical equation is given below.
  • SO3 + NaOH → NaHSO4

Uses of Sulfur Trioxide – SO3

  • Used as a bleaching agent to remove residual hydrogen peroxide, or as a digesting agent to separate pulp from lignin.
  • Used as a catalyst in the oxidation reaction of sulfur dioxide to sulfur trioxide.
  • It is a strong inorganic acid mists containing sulfuric acid used in industry or in production of commercial products.
  • Used as an essential reagent in sulfonation reaction.
  • Used in the manufacture of photoelectric cells and solar energy devices.

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