Animated Solution for Chemistry - Salt Analysis: In the following reaction sequence in aqueous soluiton, the species X, Y and Z respectively, are –

The reaction of thiosulphate ions with silver ions is one of the most visually striking and chemically rich sequences in qualitative salt analysis. It’s a beautiful dance of complexation, precipitation, and decomposition. Let’s break down this sequence step by step to uncover the identities of species X, Y, and Z.

When we first add silver ions (${\text{Ag}}^{+}$) to a solution containing thiosulphate ions (${\text{S}}_2{\text{O}}_3^{2-}$), we might expect an immediate precipitate, as silver forms many insoluble salts. However, the solution remains perfectly clear!

Why does this happen? Thiosulphate is an excellent ligand. Instead of precipitating, it wraps around the silver ion to form a highly stable, soluble coordination complex known as the dithiosulfatoargentate(I) ion.

This clear solution is our species X. Interestingly, this exact chemical property is utilized in black-and-white photography, where sodium thiosulphate (commonly known as "hypo") is used to dissolve unexposed silver bromide from the photographic film, "fixing" the image.

White Precipitate

As we continue to add more silver ions to the solution, we eventually reach a tipping point. The excess ${\text{Ag}}^{+}$ ions disrupt the equilibrium of the soluble complex. The concentration of silver ions becomes high enough to exceed the solubility product of silver thiosulphate.

The complex breaks down, and a distinct white precipitate of silver thiosulphate (${\text{Ag}}_2{\text{S}}_2{\text{O}}_3$) crashes out of the solution. This white precipitate is our species Y.

Fading to Black

Here is where the chemistry gets really interesting. The white precipitate of silver thiosulphate is notoriously unstable in an aqueous environment. You don't even need to add any more reagents; simply letting it stand for some time (or gently warming it) will cause a dramatic color change.

The silver thiosulphate undergoes a decomposition reaction, breaking down into silver sulphide and sulphuric acid.

Silver sulphide (${\text{Ag}}_2\text{S}$) is highly insoluble and appears as a dense black precipitate. This final, stable black solid is our species Z.

By tracing the visual clues—from a clear solution to a white precipitate, and finally to a black precipitate—we have successfully identified all three species. X is $[\text{Ag}({\text{S}}_2{\text{O}}_3{)}_2{]}^{3-}$, Y is ${\text{Ag}}_2{\text{S}}_2{\text{O}}_3$, and Z is ${\text{Ag}}_2\text{S}$.