Sulfur Trioxide - Structure of Solid SO₃

Structure of Solid SO₃

The nature of solid SO₃ is a surprisingly complex area because of structural changes caused by traces of water. Upon condensation of the gas, absolutely pure SO₃ condenses into a trimer, which is often called γ-SO₃. This molecular form is a colorless solid with a melting point of 16.8 °C. It adopts a cyclic structure described as ₃.

If SO₃ is condensed above 27 °C, then α-SO₃ forms, which has a melting point of 62.3°C. α-SO₃ is fibrous in appearance, like asbestos (with which it has no chemical relationship). Structurally, it is the polymer _n. Each end of the polymer is terminated with OH groups. β-SO₃, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5 °C. Both the gamma and the beta forms are metastable, eventually converting to the stable alpha form if left standing for sufficient time. This conversion is caused by traces of water.

Relative vapor pressures of solid SO₃ are alpha < beta < gamma at identical temperatures, indicative of their relative molecular weights. Liquid sulfur trioxide has vapor pressure consistent with the gamma form. Thus heating a crystal of α-SO₃ to its melting point results in a sudden increase in vapor pressure, which can be forceful enough to shatter a glass vessel in which it is heated. This effect is known as the "alpha explosion".

SO₃ is aggressively hygroscopic. In fact, the heat of hydration is sufficient that mixtures of SO₃ and wood or cotton can ignite. In such cases, SO₃ dehydrates these carbohydrates.