Alkenes: Unsaturated hydrocarbons

A hydrocarbon is a compound made of hydrogen and carbon only. Hydrocarbons split into two families you must be able to tell apart:

• Alkanes — every bond is a single bond. We say they are SATURATED: every carbon already holds as many hydrogen atoms as it possibly can.
• Alkenes — they contain a C=C double bond. We say they are UNSATURATED: the double bond means the molecule is not holding the maximum number of hydrogen atoms; there is "room" to add more atoms.

Think of "saturated" the way a sponge is saturated with water — it is completely full and cannot take any more. A saturated hydrocarbon cannot take any more hydrogen. An unsaturated hydrocarbon still can.

The smallest alkene is ethene (two carbons joined by a C=C). You need at least two carbons to have a carbon-to-carbon double bond, so there is no one-carbon alkene.

The headline you must remember: the difference between the two families is the C=C double bond. Everything else in this subtopic — the reactivity and the bromine water test — flows directly from that single feature.

Alkanes are fairly unreactive — their carbon atoms are joined by strong single bonds that are hard to break, so alkanes mostly just burn.

Alkenes are MORE reactive, and the reason is always the same: the C=C double bond can OPEN UP.

• One of the two bonds in the C=C can break, leaving each carbon with a "spare" bonding position.
• A small molecule (such as bromine) then adds across the old double bond — one atom joins each carbon.
• The C=C double bond becomes a C–C single bond, and the molecule has gained new atoms.

This is called an addition reaction. Alkanes cannot do this: they have no double bond to open up, so there is nowhere for the extra atoms to go.

The exam-winning sentence:

"Alkenes are more reactive than alkanes because of the C=C double bond, which can open up to add atoms across it in an addition reaction."

Notice what this means for marks: if a question asks why alkenes are more reactive, the answer is the C=C double bond (and that it can open up / undergo addition). An answer that just says "alkenes are reactive" or "they have more bonds" does not earn the mark.

The standard chemical test for a C=C double bond (unsaturation) is the bromine water test. It is one of the most frequently examined tests in IGCSE Chemistry, so learn the procedure, the observations and the exact colour change.

Procedure. Add a few cm³ of bromine water (an orange-brown solution of bromine in water) to a small sample of the hydrocarbon and shake.

Result with an ALKENE (unsaturated):

• The bromine water is DECOLOURISED — it changes from orange-brown → colourless.
• This happens because the bromine adds across the C=C double bond (the double bond opens up). The bromine is used up, so its colour disappears.

Result with an ALKANE (saturated):

• The bromine water stays orange-brown — no change.
• An alkane has no C=C double bond for the bromine to add across, so no reaction happens.

The exact colour change — get this right.

The mark scheme wants orange-brown → colourless. Two very common ways students throw away the mark:

• Writing "turns clear" or "goes clear". "Clear" means transparent — but orange-brown bromine water is already transparent (you can see through it). The point is that the colour disappears, so the right word is colourless (or "decolourised").
• Forgetting to say what the alkane does. A full answer states both: the alkene decolourises it, and the alkane leaves it orange-brown (unchanged). That contrast is what proves the test distinguishes them.

Putting the test to use — identifying an unknown.

A typical exam question gives you two colourless hydrocarbons (one alkane, one alkene) and asks how to tell them apart:

• Add bromine water to each.
• The one that turns the bromine water from orange-brown → colourless is the alkene (unsaturated).
• The one that leaves the bromine water orange-brown is the alkane (saturated).

This subtopic is small, so the marks come down to precise wording. Lock in these three phrases:

1. Why alkenes are more reactive: "Because of the C=C double bond, which can open up to add atoms across it."

2. The test: "Add orange-brown bromine water and shake."

3. The observations: "An alkene decolourises it (orange-brown → colourless); an alkane leaves it orange-brown (no change)."

Traps that lose marks every series:

• "Goes clear" instead of "colourless" — clear ≠ colourless. Bromine water is already clear (transparent); it is the colour that vanishes.
• Naming the colour wrongly — bromine water is orange-brown, not "yellow", not "red", not "purple" (purple is iodine).
• Only describing the alkene result and not the alkane — you must contrast the two to identify an unknown.
• Saying the alkane "stays clear" — say it stays orange-brown (unchanged).
• Confusing this with the alkane reaction with bromine in UV light — that is a substitution of a gas, not the bromine water test, and it is not what distinguishes alkenes here.

If you can write those three phrases with the exact colours, you will pick up essentially all the marks this subtopic offers.