Hydrogenation of vegetable oils using nickel as a catalyst is an example of:

In an addition reaction, two or more molecules combine to form a single larger product, without losing any atoms. Addition reactions are typical of unsaturated compounds, which have double or triple bonds between carbon atoms. During the reaction, the double or triple bond opens up, and new atoms attach to the carbon atoms.

Vegetable oils, such as groundnut oil, cottonseed oil and soyabean oil, contain unsaturated fatty acids with carbon-carbon double bonds. This is why they are liquid at room temperature. In hydrogenation, hydrogen gas is passed through the heated oil in the presence of finely divided nickel (or palladium) as a catalyst. The hydrogen atoms add across the double bonds, turning them into single bonds. The oil becomes saturated and changes into a semi-solid fat. This is how vanaspati ghee (a hydrogenated vegetable fat) and margarine are made.

The reaction can be shown simply as: R-CH=CH-R + H2 (nickel catalyst) → R-CH2-CH2-R.

Hydrogenation makes fats last longer without going rancid and gives them a texture like butter or ghee. However, partial hydrogenation produces trans fats, which raise "bad" (LDL) cholesterol and increase the risk of heart disease. For this reason, the Food Safety and Standards Authority of India (FSSAI) has limited trans fats in oils and fats to 2% from 2022. Doctors advise that oils with unsaturated fats are healthier than saturated animal fats.

Why not the other options?

• ✗Substitution reactions replace one atom or group with another; they are typical of saturated compounds such as alkanes (for example, the chlorination of methane in sunlight).
• ✗Decomposition reactions break one compound into simpler substances.
• ✗Displacement reactions occur when a more reactive element displaces a less reactive one from its compound.