The yellow solid separated out could be Lead iodide (PbI2). When a solution of a lead compound, such as lead nitrate (Pb(NO3)2), is added to a solution of potassium iodide (KI), a chemical reaction takes place, resulting in the formation of lead iodide (PbI2) as a yellow precipitate.

The reaction can be represented as follows:

```

Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

```

Lead iodide is insoluble in water and has a bright yellow color, which is why it appears as a yellow solid when the two solutions are mixed. This reaction is commonly used as a qualitative test for the presence of lead ions in a solution.