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. Tollens' reagent.
2. Acidified potassium dichromate.

Both 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.