Let's identify an unknown sample (Propanal vs Propanone). Step 1: Take two test tubes. Label one A and one B. Step 2: Add Tollens' reagent to both. Step 3: Heat them gently in a water bath. Step 4: Watch for the result. Tube A (Propanal - an aldehyde) will form a shiny, metallic silver layer on the inside wall of the glass. Tube B (Propanone - a ketone) will show no change and remain a colorless liquid. Conclusion: Tube A is the aldehyde.