Formaldehyde can be distinguished from acetaldehyde by the use of

The presence of a methyl ketone OR Methylated alcohol(at the terminal position) is confirmed by performing this test. The compound is heated in the presence of base and iodine or bromine. The product iodoform or bromoform is yellow and has a characteristic odour. This confirms the presence of methylated ketone. Acetaldehyde has Me-C=O, Hence it gives positive test. Ethanol also gives the test.