Question: Give a simple chemical test to distinguish between Ethanal and Propanone.

Step 1 (Governing Principle): [0.5 Mark]
Ethanal ($CH_3CHO$) is an aldehyde, whereas Propanone ($CH_3COCH_3$) is a ketone. Aldehydes can be distinguished from ketones using the Tollens' Test. Tollens' reagent is ammoniacal silver nitrate solution, $[Ag(NH_3)_2]^+ OH^-$. Aldehydes reduce Tollens' reagent to metallic silver, forming a "silver mirror," while ketones do not react.

Step 2 (Chemical Reaction): [0.25 Mark]
The reaction for Ethanal with Tollens' reagent is as follows:

$$CH_3CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow CH_3COO^- + 2Ag(s) + 4NH_3 + 2H_2O$$

Step 3 (Final Concluded Answer): [0.25 Mark]

• Ethanal: Gives a silver mirror on the inner walls of the test tube due to the formation of metallic silver ($Ag$).
• Propanone: Shows no reaction; no silver mirror is formed.

💡 EL10 Concept Insight: Think of the Tollens' test like a "detective" for aldehydes; because aldehydes are easily oxidized, they "steal" the silver from the reagent and paint it onto the glass, while ketones are too stable to let that happen!