Misconception 1: "Alkenes burn cleaner than alkanes"
This is false. Alkenes actually burn with a smokier flame than alkanes because they have a higher proportion of carbon relative to hydrogen (CₙH₂ₙ vs CₙH₂ₙ₊₂). The higher carbon content means incomplete combustion is more likely. Alkanes are better fuels for clean combustion. The special value of alkenes is in addition reactions, not combustion.
Misconception 2: "The formula CₙH₂ₙ means alkenes have n hydrogens"
No — CₙH₂ₙ means the number of hydrogens is 2 times the number of carbons. For ethene (n=2): 2×2 = 4 hydrogens, so C₂H₄. For propene (n=3): 2×3 = 6 hydrogens, so C₃H₆. Don't confuse the subscript n with the value 2n.
Misconception 3: "Decolourisation of bromine water proves oxygen is present"
Decolourisation of bromine water proves the presence of a C=C double bond (an unsaturated compound), not oxygen. The bromine reacts with the double bond in an addition reaction. This test identifies alkenes, not oxidising agents.
This common misconceptions covers Common Misconceptions 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 10 of 14 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Alkenes
What is the general formula for alkenes?
Explain what is meant by the term 'unsaturated' when applied to alkenes.