ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

Sulfite ion (SO3^2-) typically forms soluble salts with many cations, particularly those of alkali metals and ammonium ions. The solubility of sulfites in water generally increases with the charge and size of the ions involved.

Ammonium and potassium sulfites, for example, are well-known to be soluble due to the presence of the ammonium ion (NH4+) and the cation from potassium (K+), both of which easily dissolve in aqueous solutions. Calcium sulfite (CaSO3) is also soluble to some extent.

Lead, on the other hand, forms lead(II) sulfite (PbSO3), which is largely insoluble in water. This is due to the relatively low solubility of many lead compounds and particularly the properties of lead sulfite, which results in precipitation rather than dissolution.

Understanding the solubility rules for sulfites and the behavior of ions in water helps clarify why lead is the correct choice as the ion that is not soluble with sulfite compared to the other ions listed.