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Write a chemical test to distinguish between formaldehyde and acetaldehyde.

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Answer
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Hint: To distinguish one from another use reactions where a change of physical characteristics happens. For example, change of color of the solution, precipitation takes place, generation of effervescence, etc.

Complete step by step answer:
Structures of formaldehyde and acetaldehyde are
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When formaldehyde and acetaldehyde are treated with iodine in NaOH. Acetaldehyde gives yellow color precipitation. But on the other hand, formaldehyde does not react with it.
From this reaction, a change of physical characteristic happens which can be visualized to understand the difference between formaldehyde and acetaldehyde.
The overall reaction is shown below.
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The mechanism is shown below,
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This reaction is commonly known as the iodoform reaction.
Iodoform reaction is used to prove the presence of \[R - CO - C{H_3}\] a group of carbonyl and \[R - CH\left( {OH} \right) - C{H_3}\] alcohol. In this reaction oxidation of the carbonyl group and alcohol group takes place. By oxidation, they form a carboxylic acid. And besides this, yellow precipitation of iodoform takes place in this reaction. This yellow precipitation is the indication that the reaction occurred.

Note:
When chlorine is used instead of iodine then it is called a chloroform reaction. In this reaction precipitation of white chloroform is formed. And when bromine is used instead of iodine or chlorine then it is called a bromoform reaction. In this reaction precipitation of pale yellow bromoform is formed. These types of reactions are collectively known as haloform reactions.