Liquid vegetable oils are converted to solid margarine by the use of

The correct answer is A, hydrogen gas. Vegetable oils are hardened into margarine and vanaspati ghee by hydrogenation. Liquid vegetable oils are unsaturated: their fatty-acid chains contain carbon-carbon double bonds. When hydrogen gas is passed through the heated oil in the presence of a finely divided nickel catalyst, hydrogen adds across these double bonds, an addition reaction, and the oil becomes saturated. Saturated fats have higher melting points, so the product is solid or semi-solid at room temperature. Partial hydrogenation can also form trans fats, which is why health experts advise limiting such fats. Option B is wrong because chlorine does not harden oils; it is used for bleaching and disinfecting water. Option C is wrong because carbon dioxide does not react with the double bonds of oil; it is used in fizzy drinks and fire extinguishers. Option D is wrong because oxygen makes oils go rancid by oxidation instead of turning them into solid fat. Exam tip: oil plus hydrogen, with a nickel catalyst, gives vanaspati or margarine.