Give scientific reasons : The hydrogenation of vegetable oil in presence of nickel catalyst forms Vanaspati-ghee.

Answer: Hydrogen adds across the carbon–carbon double bonds of the unsaturated vegetable oil, converting it into a saturated fat which is semi-solid (vanaspati ghee).

Vegetable oils are unsaturated compounds: they are esters (triglycerides) whose fatty-acid chains contain carbon–carbon double bonds, and so they are liquid at room temperature. When hydrogen gas is passed through the heated oil in the presence of a nickel catalyst, hydrogen adds to these double bonds (an addition reaction) and the chains become saturated:

\[\mathrm{{>}C{=}C{<} + H_2 \xrightarrow{\text{Ni}} {>}CH{-}CH{<}}\]

\[\text{unsaturated vegetable oil (liquid)} + H_2 \xrightarrow{\text{Ni catalyst}} \text{saturated fat (vanaspati ghee, semi-solid)}\]

The saturated fat has a higher melting point and is semi-solid, so this hydrogenation process is used to make vanaspati ghee from vegetable oil.