Solutions of Fe²⁺ ions and Ag⁺ ions react according to the following equation:

Fe²⁺(aq) + Ag⁺(aq) _↓  Fe³⁺(aq) + Ag(s)

The reverse reaction can also be carried out:

Fe³⁺(aq) + Ag(s) _↓  Fe²⁺(aq) + Ag⁺(aq)

Solutions of iron(II) sulfate and silver nitrate were mixed and left overnight. The mixture was filtered and the filtrate tested for the presence of Fe²⁺, Ag⁺ and Fe³⁺ ions. All three tests were positive.

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How might you test for the presence of Fe²⁺ ions?

You could add sodium hydroxide solution. A green precipitate shows that Fe²⁺ ions are present. The problem is that Fe³⁺ ions also give a precipitate with sodium hydroxide so the result may be difficult to interpret. It would be better to test with potassium thiocyanate solution because only Fe²⁺ ions react. They produce a red colour.

How might you test for the presence of Fe³⁺ ions?

You could add sodium hydroxide solution. A brown precipitate shows that Fe³⁺ ions are present. The problem is that Fe²⁺ ions also give a precipitate with sodium hydroxide so the result may be difficult to interpret. It would be better to test with potassium hexacyanoferrate(II) solution because only Fe³⁺ ions react. They produce a blue colour.

How might you test for the presence of Ag⁺ ions?

Add a solution containing chloride ions such as hydrochloric acid because this gives a white precipitate.

To explore testing for ions in more detail subscribe to the Analytical chemistry course (Chemical tests).

Some reactions don't go to completion, with all reactants converted to products. Instead, some of the products react together to re-form the reactants. The forward and reverse reactions continue indefinitely. We say that an equilibrium has been established containing both reactants and products. 

We show that an equilibrium has been formed by writing an equation with a double arrow pointing in both directions. This means that both forward and reverse reactions are happening at the same time:

Fe²⁺(aq) + Ag⁺(aq) _∏  Fe³⁺(aq) + Ag(s)