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.
Topic position
Section 8 of 14
Practice
20 questions
Recall
15 flashcards
📖 Key Definitions
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.