What makes an alkene different from an alkane, and how does the bromine water test tell them apart?
Describe the alkenes as unsaturated hydrocarbons, use the bromine water test to distinguish them from alkanes, and describe a simple addition reaction
A focused answer to the N(A) Chemistry outcome on alkenes. The carbon double bond that makes alkenes unsaturated, the bromine water test, and what happens in a simple addition reaction.
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What this dot point is asking
The syllabus wants you to describe the alkenes as unsaturated hydrocarbons, to use the bromine water test to tell them apart from alkanes, and to describe a simple addition reaction. The single most important fact is that alkenes contain a carbon-to-carbon double bond, and that double bond explains both the test result and why alkenes react by addition.
The answer
What makes an alkene
The alkenes are hydrocarbons (hydrogen and carbon only) that contain a carbon-to-carbon double bond. The simplest is ethene, . Because of the double bond they are described as unsaturated, meaning they do not have the most hydrogen atoms possible: the double bond could open to take on more.
Alkanes versus alkenes
The difference is the bond:
- alkanes are saturated (only single bonds), so they are fairly unreactive,
- alkenes are unsaturated (one carbon-to-carbon double bond), so they are more reactive.
The bromine water test
The bromine water test distinguishes the two families:
- add orange bromine water and shake,
- with an alkene, the bromine water turns colourless (it is decolourised),
- with an alkane, the bromine water stays orange (no change).
So decolourising orange bromine water is the test for an unsaturated compound (an alkene).
Addition reactions
The double bond lets an alkene take part in an addition reaction: the double bond opens up so new atoms add on across it, turning the alkene into a saturated product. For example, ethene reacts with hydrogen to form ethane, and with bromine to form a colourless product (which is why bromine water is decolourised). An alkane has no double bond to open, so it cannot react this way.
Examples in context
Example 1. Making margarine. Liquid vegetable oils contain carbon-to-carbon double bonds. Adding hydrogen across these double bonds (an addition reaction) turns the oil into a solid fat, which is how margarine is made, a direct industrial use of alkene chemistry.
Example 2. Checking for unsaturation in foods. Food scientists use the idea behind the bromine water test to measure how unsaturated a fat or oil is. The more double bonds, the more reagent it reacts with, showing how a simple test scales up to real analysis.
Try this
Q1. State what is meant by an unsaturated hydrocarbon. [1 mark]
- Cue. A hydrocarbon that contains a carbon-to-carbon double bond.
Q2. Describe what you would observe when ethene is shaken with orange bromine water, and what this shows. [2 marks]
- Cue. The bromine water turns colourless (is decolourised), which shows ethene is unsaturated (has a double bond).
Q3. Explain why ethene reacts with hydrogen in an addition reaction but ethane does not. [2 marks]
- Cue. Ethene has a double bond that can open to let hydrogen add on; ethane has only single bonds, so there is no double bond to open.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksEthene () is an alkene. (a) State what is meant by unsaturated. (b) Describe the result of adding ethene to orange bromine water. (c) State what you would see if you added an alkane such as ethane to bromine water instead.Show worked answer →
(a) Unsaturated means the molecule contains a carbon-to-carbon double bond.
(b) The orange bromine water turns colourless (it is decolourised) when shaken with ethene.
(c) With an alkane such as ethane, the bromine water stays orange (no change), because alkanes have no double bond.
What markers reward: unsaturated as having a double bond, bromine water going from orange to colourless with the alkene, and staying orange with the alkane.
Original4 marksEthene reacts with hydrogen to form ethane. (a) State the name for this type of reaction. (b) Explain, in terms of the double bond, why an alkene can take part in this reaction but an alkane cannot.Show worked answer →
(a) It is an addition reaction.
(b) The alkene has a carbon-to-carbon double bond. In an addition reaction the double bond opens up so new atoms can add on. An alkane has only single bonds, with no double bond to open, so it cannot react in this way.
What markers reward: addition (reaction), and that the double bond opens to let atoms add on, which the alkane lacks.
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