Sulfur trioxide

Sulfur trioxide (SO₃) is an oxide of sulfur that reacts vigorously with water to form sulfuric acid. It exists as a gas, liquid, or solid depending on temperature and molecular arrangement.

Sulfuric acid: A strong, diprotic mineral acid with formula H₂SO₄, produced industrially from sulfur trioxide and water. Adding water to SO₃ produces this acid, often with intense heat and mist formation.
Contact process: The principal industrial process for manufacturing sulfuric acid by oxidizing sulfur dioxide and absorbing sulfur trioxide. Its catalytic oxidation stage produces SO₃ at large scale.
Sulfur: A nonmetallic chemical element with atomic number 16, occurring in minerals and living organisms. Sulfur is the central atom in SO₃ and determines its oxidation chemistry.
Acid rain: Precipitation made unusually acidic by sulfur and nitrogen oxides transformed in the atmosphere. Atmospheric SO₃ reacts with water and contributes to sulfuric acid formation in acidic aerosols and precipitation.
Oleum: A solution of sulfur trioxide in sulfuric acid, used as a concentrated source of SO₃. SO₃ dissolves in sulfuric acid to form this transportable, highly reactive mixture.
Vanadium(V) oxide: An inorganic compound, V₂O₅, used as a catalyst in industrial oxidation reactions. It catalyzes the conversion of sulfur dioxide and oxygen into SO₃ in the contact process.
Oxygen: A reactive nonmetallic element with atomic number 8 that forms oxides with most elements. Three oxygen atoms bonded to sulfur make up each SO₃ molecule.
Sulfuric acid mist: A suspended aerosol of tiny sulfuric acid droplets that can form when sulfur trioxide meets humid air. Rapid hydration of SO₃ can create corrosive mist that is difficult to contain.
Sulfur trioxide equilibrium: The reversible gas-phase reaction in which sulfur dioxide and oxygen form sulfur trioxide. The equilibrium governs how much SO₃ forms during industrial oxidation.
Sulfonation: A chemical reaction that introduces a sulfonic acid group into an organic molecule. SO₃ is a potent reagent for adding sulfonic acid groups to aromatic compounds.

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