Preparation of iodine solution
Since $\ce{ I_2}$ is a non-polar covalent molecule, it does not ionize in water. It  cannot also be soluble in the polar water.

$\ce{KI}$, which is a polar, ionic compound, will ionize and dissolve in water. When KI dissolves in water, it ionizes to $\ce{K^+}$ and $\ce{I^-}$.

The $\ce{I^-}$ will react with $\ce{ I_2}$ to form the complex ion $\ce{I_3^-}$. $\ce{I_3^-}$ being negatively charged will dissolve in water.