What is a silver mirror test? Explain with reaction.

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Hint: The silver mirror test is a qualitative test used to distinguish between carbonyl groups i.e. aldehydes and ketones. It is also called Tollens' test after the name of the scientist.

Complete answer:
It is a qualitative laboratory test which is used to distinguish between an aldehyde and a ketone. The reagent used for this test is Tollens’ reagent which has silver ions coordinated to ammonia ions. The solution is colourless initially.
Step 1:- The aqueous silver nitrate is taken in the test tube. Added aqueous sodium hydroxide solution to it. The reaction for this step is :

A g N O_{3} + N a O H \to A g O H + N a H O_{3}

2 A g O H \to A g_{2} O + H_{2} O

Step 2 :- To this precipitate of

A g_{2} O

(silver oxide), aqueous ammonia is added drop wise till we get these ppt completely dissolved.

A g_{2} O + 4 N H_{3} + H_{2} O \to 2 A g {(N H_{3})_{2}}^{+} + 2 O H^{-}

Step 3 :- These hydroxide ions produced in step 2 are used to oxidize the group. If the compound given is aldehyde; it readily gets oxidized while if the compound contains a ketonic group; then it is not hydrolyzed.

C H_{3} C H O + 2 A g^{+} + 2 O H^{-} \to C H_{3} C O O H + 2 A g + H_{2} O

Further, the silver ions are reduced into metallic silver. This metallic silver forms a mirror in a clean test tube. Thus, the test is named the silver mirror test.

Note: If the ketonic group is present in the solution; then no silver mirror is formed. We get a clear solution.
This test is based on the principle that aldehydes are easily oxidized while ketones are not.
If there are alpha-hydroxy ketones in the test tube then also the test shows positive. This is because these can tautomerize into an aldehyde.