Question

A solution is saturated with silver sulfate and contains excess solid silver sulfate: Ag2SO4(?) ⇌ 2Ag + (??) + SO4^2- (??)

A small amount of solid silver sulfate containing a radioactive isotope of silver is added to this solution. Within a few minutes, a portion of the solution phase is sampled and tests positive for radioactive Ag+ ions. Explain this observation.

Answer 1

Silver sulfate is sparingly soluble in water (sparingly soluble ionic solid). However, when we add an excess of solid Ag₂SO₄ to water, it dissolves to a small extent and produces a mixture consisting of a very dilute solution of Ag⁺ and ions in equilibrium with undissolved silver sulfate. The equilibrium can be written as follows

Ag,S04 (s) = Ag+ (aq) + soś (aq)

This is a dynamic equilibrium which means that during this equilibrium some of the solid Ag₂SO₄ dissolves to form Ag⁺ and and at the same time some of the Ag⁺ and ions in the solution recombine to precipitate Ag₂SO₄ in solid form. At equilibrium both these opposite process (dissolution and precipitation) have equal rate. So if we add a radioactive solid Ag₂SO₄ to the solution it participates in this dynamic equilibrium, i.e. some of this solid radioactive Ag₂SO₄ dissolves into the solution phase as Ag⁺ and and the radioactive Ag⁺ thus formed gives a positive result for the test for radioactivity.