Can an alkene ever “act like” an alkane?

Yes, and this shows up a lot in IB Chemistry reaction pathways. Once an alkene undergoes an addition reaction, the product may no longer contain a C=C bond. At that point, the product is saturated in that region and will be less reactive in the specific ways alkenes are reactive. For example, adding Br₂ across an alkene gives a dibromoalkane with only single bonds between the original carbons. However, this doesn’t mean the original alkene wasn’t an alkene; classification depends on the functional group present at the time. In explanations, state clearly that the presence of C=C defines alkene behavior.

Are all compounds with a double bond automatically alkenes?

Not automatically, and IB Chemistry expects you to be precise. Alkenes are hydrocarbons with a carbon-carbon double bond (C=C) in a non-aromatic system. A compound could have a double bond between different atoms (like C=O), which would place it in a different family entirely. There are also aromatic compounds where bonding is delocalized, and they do not behave like simple alkenes in typical addition tests. In the simplest IB-level distinction questions, though, if the hydrocarbon has a clear C=C, the intended answer is “alkene.” When in doubt, identify the functional group first and then match the homologous series.

Why doesn’t an alkane decolorize bromine water in the dark?

Because the key reactive feature is missing, and IB Chemistry wants you to say what that feature is. Alkanes have only σ bonds, which are strong and not electron-rich in an exposed way. Without UV light, bromine molecules do not readily form radicals, so the substitution pathway doesn’t initiate. Alkenes, however, have a π bond with accessible electron density that can interact with bromine, allowing addition to occur readily. So the bromine water test is really a test for unsaturation, not “orange liquid magic.” A great exam phrase is: “No π bond, no rapid reaction under these conditions.”

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IB Chemistry: How to Tell Alkanes vs Alkenes Fast

IB Chemistry

RevisionDojo Team

•Last updated: 19 Jul 2026•7 min read

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IB Chemistry: How to Tell Alkanes vs Alkenes Fast

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.

Two bottles, one test, one stressed student

The 20-second checklist (exam-friendly)

When you’re under time pressure in IB Chemistry, run this quick check:

Look for C=C: if there’s a double bond, it’s an alkene.
No multiple bonds: only single C-C bonds means alkane.
Use bromine water: alkene decolorizes, alkane stays orange/brown (in the dark).
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

Only single carbon-carbon bonds (C-C)
Saturated hydrocarbon (maximum H atoms)
General formula: CₙH₂ₙ₊₂ (for acyclic alkanes)

What makes an alkene an alkene

At least one carbon-carbon double bond (C=C)
Unsaturated hydrocarbon (fewer H atoms than the corresponding alkane)
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:

A C=C double bond contains one sigma (σ) bond and one pi (π) bond.
The π bond is more exposed (electron density above and below the plane).
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

Bromine water starts orange/brown
It decolorizes (orange to colourless)
This is an addition reaction across the C=C

Alkane + bromine water

No reaction in the dark
The solution stays orange/brown
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)

Alkenes react (purple solution decolorizes and may form brown MnO₂)
Alkanes: no reaction under normal conditions

Infrared (IR) spectroscopy

Alkenes show a C=C stretch around ~1650 cm⁻¹
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)

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.
Remember geometry: alkane carbons are tetrahedral (~109.5°). Alkene carbons at the double bond are trigonal planar (~120°). This can support explanation questions.
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.

IB Chemistry: How to Tell Alkanes vs Alkenes Fast