Question

Give one chemical test to distinguish between formaldehyde and acetaldehyde.

Hint:

Acetaldehyde is the only aldehyde that gives a positive iodoform test!

Answer 1

Concept:
The iodoform test is specific to compounds containing a methyl ketone group (CH₃CO-) or an alcohol that can be oxidized to one.

Formaldehyde (HCHO): Does not have a methyl group attached to the carbonyl carbon.
Acetaldehyde (CH₃CHO): Contains the required methyl group.

Performing the Test:
When reacted with iodine (I₂) in the presence of sodium hydroxide (NaOH):

Acetaldehyde gives a yellow precipitate of iodoform (CHI₃).
CH₃CHO + 3 I₂ + 4 NaOH → CHI₃ ↓ (yellow) + HCOONa + 3 NaI + 3 H₂O

Formaldehyde does not react and gives no precipitate.