Sulfur trioxide - SO3
Sulfur trioxide is an anhydride to sulfur acid and in pure form it is a fluid. Gaseous sulfur trioxide is highly irritating to the eyes, throat and respiratory tract.
It reacts with water and water vapour. Without water it’s not particularly reactive to metals.
Molecular weight 80.066 g/mol
Solid phase Melting point: 16.9 °C
Liquid phase Boiling point: 45 °C