Is SO2 a liquid, a solid, or a gas at room temperature?

1 Answer

#"SO"_2# is a gas at room temperature.


A gas.

You can start simply by thinking about #SO_2# having covalent bonds and a simple molecular structure like #CO_2#.

To go into a little more depth, we need to think about what type(s) of intermolecular forces #SO_2# has. Like all simple molecules it will have the weakest intermolecular forces, Van der Waals.

It is also a polar molecule, because it has a bend shape as a result of having two (double) binding pairs and a lone pair on the central S atom. The S atom is less electronegative that O, so the S-O bonds are polar and not symmetrically opposed, so the dipoles don't cancel each other out and we have permanent dipole-dipole intermolecular forces which are stronger than Van der Waals, so we'd expect a higher melting and boiling point than #CO_2#.

Because it doesn't have any hydrogen atoms, sulphur dioxide can't have hydrogen bonding - the strongest intermolecular force. So, in summary we are expecting a low melting and boiling point, but not as low as for carbon dioxide. The actual m.p. is -72 deg.C compared to a m.p. of -78.5 deg.C for carbon dioxide, and the boiling point is -10 deg.C compared to -56.6 deg.C for carbon dioxide. You can see from these figures that both are gases at room temperature.