The test for a carbonyl group is adding 2,4-dinitrophenylhydrazine (2,4-DNPH). Compounds containing a carbonyl group will produce a yellow or orange precipitate called a 2,4-dinitrophenylhydrazone. 

In practical work, 2,4-DNPH is normally used dissolved in methanol and sulfuric acid as a pale orange solution called Brady's reagent.

1. Add 2,4-DNPH to the carbonyl compound and an orange precipitate will form.
2. Filter the precipitate and purify it by recrystallisation.
3. Measure the melting point of the precipitate.
4. Compare its melting point to a database to identify the original carbonyl compound.

1. Fehling's solution.
2. Tollens' reagent.
3. Acidified potassium dichromate.

All of these tests give positive results with aldehydes, but not with ketones (due to the ability of aldehydes to be oxidised to carboxylic acids).

Tollens' reagent is a solution containing the complex ion [Ag(NH₃)₂]⁺ formed by the addition of ammonia to silver nitrate.

Aldehydes are oxidised to carboxylic acid when warmed with Tollens' reagent, which results in the formation of a silver mirror on the inside of the test tube due to the reduction of silver ions (Ag⁺) to metallic silver (Ag).

Ketones do not react with Tollens' reagent so do not produce the silver mirror.

Fehling's solution is an alkaline solution containing blue copper(II) ions.

Aldehydes are oxidised to carboxylic acids when warmed with Fehling's solution, which results in a colour change from blue to brick red due to the formation of a copper(I) oxide precipitate.

Ketones do not react with Fehling's solution so the solution remains blue.

Compounds containing the CH₃CO- group react with a warm alkaline solution of iodine to form a yellow precipitate of triiodomethane (CHI₃).

This precipitate can be identified by its melting point of 119°C.

1. The methyl ketone is halogenated and the 3 hydrogens in the CH₃ group are replaced by iodine.
2. The intermediate is hydrolysed to form triiodomethane (CHI₃).