In IB Chemistry, there’s a particular kind of panic that hits when a question shows two “just-hydrocarbons” and asks you to identify which is which. The molecules look polite. The names look familiar. And yet you can feel the marks slipping away because you can’t justify your choice.
Here’s the calm truth: distinguishing alkanes vs alkenes in IB Chemistry is one of those topics that becomes simple once you know what to look for -- and what to test. The bond tells the story, the test confirms it, and your explanation earns the points.
The 20-second checklist (exam-friendly)
When you’re under time pressure in IB Chemistry, run this quick check:
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Look for C=C: if there’s a double bond, it’s an alkene.
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No multiple bonds: only single C--C bonds means alkane.
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Use bromine water: alkene decolorizes, alkane stays orange/brown (in the dark).
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Back-up tests: acidified KMnO₄ reacts with alkenes; IR can show a C=C absorption.
If you want a broader functional group refresher first, see Functional Groups Explained for IB Chemistry.
Alkanes vs alkenes in IB Chemistry: the structural difference
In IB Chemistry, the marker isn’t asking for vibes. They want a structural reason.
What makes an alkane an alkane
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Only single carbon-carbon bonds (C--C)
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Saturated hydrocarbon (maximum H atoms)
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General formula: CₙH₂ₙ₊₂ (for acyclic alkanes)
What makes an alkene an alkene
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At least one carbon-carbon double bond (C=C)
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Unsaturated hydrocarbon (fewer H atoms than the corresponding alkane)
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General formula: CₙH₂ₙ (for acyclic alkenes with one double bond)
This one change is why alkenes behave like they’re always ready to react -- because, electronically, they are.
For extra context on why different hydrocarbons behave differently, read Why Do Hydrocarbons Vary In Structure And Reactivity?.
Why the double bond changes everything (and how to explain it)
To score well in IB Chemistry, connect bonding to reactivity:
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A C=C double bond contains one sigma (σ) bond and one pi (π) bond.
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The π bond is more exposed (electron density above and below the plane).
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That exposed electron density makes alkenes much more likely to react with electrophiles.
So alkenes commonly undergo addition reactions, while alkanes (with only strong σ bonds) are relatively unreactive and tend to undergo substitution reactions, typically needing UV light.
If mechanisms are where you lose marks, these two are worth locking in:
The bromine water test (the standard IB Chemistry differentiator)
If you only memorize one lab distinction for IB Chemistry, make it this one.
Alkene + bromine water
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Bromine water starts orange/brown
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It decolorizes (orange to colourless)
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This is an addition reaction across the C=C
Alkane + bromine water
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No reaction in the dark
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The solution stays orange/brown
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Reaction only happens under UV light via radical substitution (not a quick room-condition test)
In exam wording, a high-scoring line is: “Alkenes decolorize bromine water due to addition across the C=C double bond; alkanes do not react under these conditions because they lack a π bond.”
To drill the substitution side, practice with R3.3.3 Substitution reactions with alkanes and reinforce it using the matching Substitution reactions Flashcards.
Two backup methods IB Chemistry loves
Bromine water is the headline, but backup methods make your answers feel inevitable.
Acidified potassium permanganate (KMnO₄)
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Alkenes react (purple solution decolorizes and may form brown MnO₂)
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Alkanes: no reaction under normal conditions
Infrared (IR) spectroscopy
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Alkenes show a C=C stretch around ~1650 cm⁻¹
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Alkanes do not show that specific C=C absorption
If you want a clean route through spectra questions, use:
Common traps (and how to avoid them)
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Don’t rely on boiling point: both series show rising boiling point with chain length. It’s not a clean distinguishing test in IB Chemistry.
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Remember geometry: alkane carbons are tetrahedral (~109.5°). Alkene carbons at the double bond are trigonal planar (~120°). This can support explanation questions.
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Cycloalkanes behave like alkanes in bromine water: they generally won’t decolorize bromine water in the dark.
Conclusion: make the bond do the talking
In IB Chemistry, distinguishing alkanes vs alkenes is about building a short chain of logic: structure (C=C or not) -- reactivity (addition vs substitution) -- evidence (bromine water, KMnO₄, IR). When you explain it that way, your answer sounds like chemistry, not guessing.
If you want to turn that clarity into speed, RevisionDojo is built for it: use the Questionbank to practice identification questions, Study Notes to tighten definitions, Flashcards for formulas and tests, and AI Chat when your explanation feels almost right but not exam-tight. Add Predicted Papers, Mock Exams, and Grading tools when you’re ready to simulate the real thing, and use the Coursework Library plus Tutors when you need targeted help. Your IB Chemistry marks don’t come from knowing more -- they come from knowing what to say, fast.
