Which simple chemical test is used to distinguish between ethanal & propanal?

Explanation:

Iodoform test: This test is used to distinguish between ethanal and propanal. In this test, a yellow precipitate of iodoform (CHI3) is formed when a compound containing a methyl ketone or a secondary alcohol with a methyl group is treated with an iodine and sodium hydroxide (NaOH) solution.
The reactions for ethanal and propanal are as follows:

Ethanal + I₂ + NaOH → CHI₃ + Na₂CO₃ + H₂O

Propanal + 3I₂ + 4NaOH → CHI₃ + 3NaI + 2Na₂CO₃ + 2H₂O

In the Iodoform test, the yellow precipitate of iodoform is formed only when the compound contains a methyl ketone or secondary alcohol with a methyl group.

Therefore, the test can be used to distinguish between ethanal (which is an aldehyde and does not have a methyl group) and propanal (which is an aldehyde and has a methyl group).

CH₃CHO \(\underset{\mathrm{NaOH}}{\stackrel{\mathrm{I}_2}{\longrightarrow}} \underset{\substack{\text { Yellow } \\ \text { Precipitate }}}{\mathrm{CH}_3}\) \(+\mathrm{HCOO}^{\ominus} \mathrm{Na}^{\oplus}\)

CH₃CH₂CHO \(\underset{\mathrm{NaOH}}{\stackrel{\mathrm{I}_2}{\longrightarrow}}\) No characteristic change

Ethanal gives positive iodoform test.