Question

Statement-I: HCHO and HCOOH can be distinguished by the Tollens test.
Statement-II: Silver mirror is formed when ammonical $AgN{O_3}$ is reduced.
A.Both the statements are true and statement-II is the correct explanation of statement-I.
B.Both the statements are true and statement-II is not the correct explanation of statement-I.
C.Statement-I is true but statement-II is false.
D.Statement-I is false but statement-II is true.

Answer 1

Compounds which have an aldehyde group can form a silver mirror when treated with Tollens reagent which is an ammoniacal silver nitrate solution. Point to be noted is this test is not given by ketones and therefore, we use this silver mirror test to distinguish between aldehydes and ketones.

Complete step by step answer:
Tollen’s reagent is a mild oxidizing reagent which oxidizes aldehyde groups into their respective acid functional groups and itself gets reduced from +1 oxidation state of silver to elemental form of silver i.e. zero oxidation state. The reagent contains sodium hydroxide in the solution so as to maintain a basic pH of the solution. Also, it has a short life, so it is freshly prepared in the laboratory at the time of reaction.
Aldehydes such as formaldehyde and acetaldehyde react with Tollens reagent i.e. ammoniacal silver nitrate solution in order to form a silver mirror. During this process, aldehydes themselves are oxidised to carboxylate ions and silver nitrate is reduced to black silver metal. Therefore, we can say that statement-II is absolutely correct. The chemical reaction involved in the reaction is shown below:
$R - CHO + [Ag(N{H_3})_2^ + ]O{H^ - } \to RCO{O^ - } + Ag \downarrow N{H^ + }4$
We are given formaldehyde and formic acid. Both the compounds have aldehyde group $\left( { - CHO} \right)$ in their structure, so they can exhibit silver mirror tests. As they both can form silver mirrors on adding Tollen’s reagent, we cannot distinguish between them. Therefore, statement-I is totally incorrect.

Now we can say that Statement-I is false but statement-II is true. Hence, the correct option is (D).

Note:
Aldehydes have $- CHO$ functional group as one hydrogen is bonded to the carbonyl group. They can be oxidised to carboxylate ions. But ketones have $- CO$ functional group only and no hydrogen atom attached to the carbonyl group. That is why they cannot be oxidized to carboxylate ions and do not give silver mirror tests.