Question

Assertion: Bromine in carbon tetrachloride test is to distinguish between: Ethene gas and ethane gas.
Reason: When this reaction occurs, molecular bromine is consumed, and its characteristic dark red brown colour disappears if bromine is not added in excess. The rapid disappearance of the bromine colour is a positive test for unsaturation.
A) Both assertion and reason are correct and reason is the correct explanation for assertion.
B) Both assertion and reason are correct but reason is not the correct explanation for assertion.
C) Assertion is correct but reason is incorrect.
D) Both assertion and reason are incorrect.

Answer 1

Hint: To solve this we must know that ethene gas is an alkene and ethane gas is an alkane. Ethene contains carbon-carbon double bonds whereas ethane contains carbon-carbon single bonds. The compounds containing double and triple bonds are said to be unsaturated compounds.

Complete step-by-step answer:
We are given that bromine in carbon tetrachloride test is to distinguish between ethene gas and ethane gas. We know that ethene gas is an alkene and ethane gas is an alkane. Ethene contains carbon-carbon double bonds whereas ethane contains carbon-carbon single bonds. The compounds containing double and triple bonds are said to be unsaturated compounds.

When bromine in carbon tetrachloride is added to the carbon-carbon double bond of ethene dibromoethane i.e. alkyl halide is produced. Thus, a carbon-carbon double bond gets converted to a carbon-carbon single bond.

When the reaction of bromine and ethene occurs, bromine molecule gets consumed and the characteristic dark red brown colour disappears if bromine is not added in excess. The rapid disappearance of bromine colour is a positive test for unsaturation.Thus, the bromine in carbon tetrachloride test is to distinguish between ethene gas and ethane gas. Thus, both assertion and reason are correct and reason is the correct explanation for assertion.

Thus, the correct option is (A) both assertion and reason are correct and reason is the correct explanation for assertion.

Note: Another test which is used to determine the unsaturation is known as Baeyer’s test. Unsaturated compounds on oxidation with hot alkaline $KMno_4$ give saturated compounds with carbon dioxide gas. The reaction is used to confirm the presence of unsaturation in the given sample.