Addition reaction
- CBSE · NCERT Class 10, Ch 4 Carbon and its Compounds
An addition reaction is one in which an unsaturated compound, with a double or triple bond, adds atoms such as hydrogen across that bond, and can end up saturated.
A double bond can open up and take in new atoms. When the atoms added are hydrogen, the reaction is called hydrogenation. It needs a catalyst such as nickel or palladium. A catalyst changes the speed of a reaction without being used up itself.
This reaction is used to hydrogenate vegetable oils. The oils have long unsaturated chains. Adding hydrogen with a nickel catalyst makes them saturated.
A small example
Ethene, CH₂=CH₂, has 2 carbons and 4 hydrogens joined by a double bond. Pass it with hydrogen over nickel: CH₂=CH₂ + H₂ → CH₃–CH₃. The product is ethane, C₂H₆. It has 6 hydrogens, 2 more than before, and only single bonds.
Where marks go
Question 26 of CBSE's 2026-27 Science sample paper has Ramya heat ethanol with concentrated sulphuric acid at 170°C to get A. She then passes hydrogen over A with nickel to get B. The 2026-27 Science marking scheme gives half a mark each for naming A, ethene, and B, ethane.
It then gives 1 mark for each step's equation. So 2 of the 3 marks are for equations, not names. In the second equation, H₂ is on the left and Ni is written over the arrow. See how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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