I need a single reagent that I could use to distinguish between Pb(NO3)2 solution and Hg(NO3)2 solution.

To distinguish between Pb(NO3)2 (lead nitrate) solution and Hg(NO3)2 (mercury nitrate) solution, you can use a single reagent called potassium iodide (KI).

To carry out the test, follow these steps:

1. Pour a small amount of the Pb(NO3)2 solution into a test tube.
2. Add a few drops of potassium iodide (KI) solution to the test tube containing Pb(NO3)2. If lead is present, a bright yellow precipitate of lead iodide (PbI2) will form. This indicates the presence of Pb2+ ions.
3. Similarly, pour a small amount of the Hg(NO3)2 solution into another test tube.
4. Add a few drops of potassium iodide (KI) solution to the test tube containing Hg(NO3)2. If mercury is present, no visible change will occur. This indicates the absence of Hg2+ ions.

Potassium iodide (KI) reacts with lead (Pb2+) ions to form insoluble lead iodide (PbI2), which appears as a yellow precipitate. On the other hand, mercury (Hg2+) ions do not react with potassium iodide.

By observing the formation or absence of the yellow precipitate, you can easily distinguish between the Pb(NO3)2 and Hg(NO3)2 solutions.