Question

How can aldehyde and ketones be distinguished from one another?

Answer 1

The aldehyde and ketones both are carbonyl compounds which have the presence of a carbon and oxygen double bond in it.
The aldehydic groups are terminal and the ketone groups are not, meaning the aldehydes are attached at the end of the alkyl chain.

Complete step-by-step answer: The aldehydes and the ketones are hydrocarbons which differ from each other in terms of structure and the type of bond they make with other compounds. Both of them are functional groups and have a carbonyl part. We will discuss the structure of both, separately, and then we would discuss the ways it could be distinguished.
The aldehydes are the compounds in which the carbon and oxygen atom share a double bond, and the hydrogen is attached to the same carbon, and the other free bond is for the alkyl group to which this functional group would get attached. It is represented as, $-(C=O)-H$ or $-CHO$.
As per the ketone group, these are the carbonyl compounds which have a carbon oxygen double bond, and two free bonds to which alkyl groups could get attached. It is represented as, $-(C=O)-$. Here the two free bonds are from carbon atoms and not oxygen.
The aldehyde and ketones can be distinguished by the use of tollen’s test. We use Tollen's reagent which consists of silver $(I)$ ions which are made from the solution of silver nitrate. Now, when we add some drops of sodium hydroxide to this solution, it results in the formation of precipitate of silver oxide, which is dissolved using the solution of dilute ammonia. This resulting solution is called the Tollen’s reagent. Now, we will see what happens when aldehyde and ketones are exposed to this solution.
In case of aldehyde,
$2A{{g}^{+}}+RCHO+{{H}_{2}}O\to 2Ag(s)\downarrow +RCOOH+2{{H}^{+}}$
Here, as we can see the aldehyde upon reaction with tollen’s reagent gives carboxylic acid and silver is precipitated as a side product, which is deposited on the walls of the test tube. This is why it is also known as the silver mirror test.
With ketones, no change would take place, and in aldehyde, as discussed, silver coloured precipitate would be formed. Hence distinguished.

Note: The aldehyde and ketones can be distinguished by the use of tollen’s test. The aldehyde gives a silver coloured precipitate after reacting with the tollen’s reagent.
The ketones are non-reactive towards the reagent, and so it does not give any precipitate.