Which substance chars when heated with conc. \(H_2SO_4\)?

Carbohydrate

The correct answer is option 4. carbohydrate.

When a carbohydrate is heated with concentrated sulfuric acid \((H_2SO_4)\), it undergoes a dehydration reaction. This reaction involves the removal of water molecules from the carbohydrate molecule. The sulfuric acid acts as a dehydrating agent, meaning it facilitates the removal of water.

Carbohydrates are composed of carbon, hydrogen, and oxygen atoms in specific ratios. During the dehydration reaction, the sulfuric acid removes hydroxyl groups \((-OH)\) and hydrogen atoms \((-H)\) from adjacent carbon atoms within the carbohydrate molecule. As a result, a water molecule is eliminated, leaving behind a double bond between the adjacent carbon atoms.

Repeated dehydration reactions lead to the formation of carbon-carbon double bonds within the carbohydrate molecule. These double bonds create a conjugated system of alternating single and double bonds, which imparts stability to the molecule.

Ultimately, the repeated removal of water molecules results in the formation of a carbon-rich residue or char. This char is black in color and consists mainly of carbon. The process is often used as a qualitative test for the presence of carbohydrates, as the formation of the black char is indicative of the carbohydrate's presence.