Saturated compounds have only single carbon-carbon bonds. Unsaturated compounds have at least one double bond, and that one difference explains the test result and the flame.
This lesson sits in carbon compounds. It assumes you can read formulae as in recognising basic carbon-compound groups.
How do I compare them?
Compare three things: the bonding, the bromine water result and the flame. Each follows from the double bond.
| Ethane (saturated) | Ethene (unsaturated) | |
|---|---|---|
| Formula | C2H6 | C2H4 |
| Bonding | single bonds only | one carbon-carbon double bond |
| Bromine water | stays brown, no visible change | brown colour disappears |
| Flame in air | less sooty | sootier |
Worked example: why is the ethene flame sootier?
The flame difference comes from the percentage of carbon in each molecule. Use relative atomic masses C = 12 and H = 1.
Ethane, C2H6. Mass of the molecule: 2(12) + 6(1) = 30. Carbon: 24. Percentage of carbon = 24 ÷ 30 × 100 = 80%.
Ethene, C2H4. Mass of the molecule: 2(12) + 4(1) = 28. Carbon: 24. Percentage of carbon = 24 ÷ 28 × 100 = 85.7%.
Ethene has a higher share of carbon. In the same air supply, more carbon stays unburnt and appears as soot.
The same idea answers a test question: unsaturated compounds react with bromine by adding across the double bond, which removes the brown colour.
The mistake that costs marks
The common wrong answer is “alkenes have more hydrogen, so they burn sootier”. It reverses the facts: alkenes have fewer hydrogen atoms than alkanes with the same carbon count.
| Wrong | Right | |
|---|---|---|
| Hydrogen in alkenes | more | fewer (two fewer for the same n) |
| Reason for soot | more hydrogen | higher percentage of carbon |
| Observation | “bromine water reacts” | “brown colour changes to colourless” |
Write what you see in the test tube, not only the word react.
Check yourself
Propane is C3H8 and propene is C3H6. Which one decolourises bromine water, and which has the higher percentage of carbon? Use C = 12 and H = 1.
Answer
Propene is unsaturated, so it decolourises bromine water.
Propane: mass 3(12) + 8 = 44, carbon 36, 36 ÷ 44 × 100 = 81.8%. Propene: mass 3(12) + 6 = 42, carbon 36, 36 ÷ 42 × 100 = 85.7%.
Propene has the higher carbon percentage.
What to study next
Move on to explaining polymer and fuel applications, where the double bond explains how plastics form. Use the graph evidence comparison lab for practice reading results.
For a teacher to go through observation wording with you, see online one-to-one Science tuition.