Question

A gas decolorizes Bayer’s reagent but does not react with Tollens reagent, the gas is:
A.Ethene
B.Ethyne
C.Ethane
D.Methane

Answer 1

The gas decolorizing Bayer’s reagent is ethene which does not react with Tollens reagent. The violet color of the reagent disappears with ethene. In Tollens test the positive result is the appearance of a silver mirror in the test tube or a dark grey precipitate.

Complete answer:
The Baeyer's reagent is an alkaline solution of cold potassium permanganate $(KMn{O_4})$. It is a violet-colored solution. It is a very strong oxidizing agent. Bayer’s reagent interacts with only those compounds which are double bonded and when the interaction takes place the violet color of the Baeyer's reagent decolorizes and becomes colorless. Baeyer's reagent is used in Baeyer's test which is significant in finding out whether unsaturation is present in a compound or not. In Baeyer's test when the reaction takes place $KMn{O_4}$ reacts with the double bonded compound (if a double bond is present) and forms an oxidized molecule. The violet color decolorizes and turns into brown sludge.
From the above information we know that only double-bonded molecule can react with Baeyer's reagent and in the options given above only one of the option is a double-bonded which is ethene so we can say that the correct option is ethene but it also states the answer does not react with Tollens reagent so we have to see how ethene reacts with Tollens reagent.
The positive result of the Tollens test is that there is the formation of a dark grey precipitate or a silver mirror in the test tube. When ethene reacts with Tollens reagent there is the formation of a white precipitate. This means no silver mirror appears and this implies that ethene does not react with Tollens reagent. Hence a gas that decolorizes Bayer’s reagent but does not react with Tollens reagent is ethene.
Therefore the correct option is A. Ethene

Note:
Potassium permanganate is a strong oxidizing agent. It has many applications in industries, medicines, etc. a dilute solution of potassium permanganate can be used as a treatment for cancer sores (ulcers). Baeyer's reagent is an alkaline potassium permanganate solution. It can be used for the reduction processes, detection of glucose, and oxidation processes along with the detection of double bonds.