Alkene: An unsaturated hydrocarbon with the general formula CₙH₂ₙ, containing one C=C double bond. Examples: ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈).
Unsaturated: A hydrocarbon molecule that contains one or more C=C double bonds. Because the double bond can open up to add atoms, unsaturated molecules are more reactive than saturated ones.
Addition reaction: A reaction in which atoms are added across a double bond, converting it to a single bond. No atoms are lost — all atoms from the reactants become part of the single product.
Decolourisation: When bromine water (orange) loses its colour in the presence of an alkene — because bromine undergoes an addition reaction with the C=C double bond, removing orange Br₂ from solution.
This definitions covers Key Definitions within Alkenes for GCSE Chemistry. Revise Alkenes in Organic Chemistry for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 8 of 14 in this topic. Make sure you can use the exact wording confidently, because definition marks are often lost through vague language.
Practice questions for Alkenes
What is the general formula for alkenes?
Explain what is meant by the term 'unsaturated' when applied to alkenes.