Compounds can exist together in a solution if they do not react with each other (i.e., they do not undergo a spontaneous redox reaction).

• $\text{FeCl}_3$ and $\text{SnCl}_2$: $\text{Fe}^{3+}$ acts as an oxidizing agent and $\text{Sn}^{2+}$ acts as a reducing agent, leading to a reaction: $2\text{Fe}^{3+} + \text{Sn}^{2+} \rightarrow 2\text{Fe}^{2+} + \text{Sn}^{4+}$.
• $\text{HgCl}_2$ and $\text{SnCl}_2$: $\text{Hg}^{2+}$ is reduced by $\text{Sn}^{2+}$, causing a reaction: $2\text{HgCl}_2 + \text{SnCl}_2 \rightarrow \text{Hg}_2\text{Cl}_2 + \text{SnCl}_4$.
• $\text{FeCl}_3$ and $\text{KI}$: $\text{Fe}^{3+}$ oxidizes the $\text{I}^-$ ion to iodine gas: $2\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_2$.
• $\text{FeCl}_2$ and $\text{SnCl}_2$: Both $\text{Fe}^{2+}$ and $\text{Sn}^{2+}$ are in their lower oxidation states and generally act as reducing agents. Neither is a strong enough oxidizing agent to oxidize the other. Thus, they do not react and can exist together.