Chemistry

The key here is recognizing a complex ion formation problem where you need to predict what happens when two iodide sources meet. When $\text{HgI}_2$ dissolves in excess $\text{KI}$, the iodide ions don't just sit passively—they coordinate around the mercury center, forming a soluble complex. The trick is understanding that while $\text{HgI}_2$ is sparingly soluble on its own, the abundance of $\text{I}^-$ ions shifts the equilibrium dramatically by creating a stable $[\text{HgI}_4]^{2-}$ complex, making the mixture clear instead of the yellow precipitate you'd expect.

The key here is recognizing a complex ion formation problem where you need to predict what happens when two iodide sources meet. When $\text{HgI}_2$ dissolves in excess $\text{KI}$, the iodide ions don't just sit passively—they coordinate around the mercury center, forming a soluble complex. The trick is understanding that while $\text{HgI}_2$ is sparingly soluble on its own, the abundance of $\text{I}^-$ ions shifts the equilibrium dramatically by creating a stable $[\text{HgI}_4]^{2-}$ complex, making the mixture clear instead of the yellow precipitate you'd expect.