WASSCE SHS
Alkanes are saturated hydrocarbons. Alkenes contain a carbon to carbon double bond and are unsaturated. Bromine water tells them apart.
A hydrocarbon contains only carbon and hydrogen. In an alkane every carbon to carbon link is a single bond, so the molecule is saturated. The general formula of a straight alkane is CₙH₂ₙ₊₂. Methane is CH₄. Ethane is C₂H₆. They burn in air. They do not decolourise bromine water at room temperature, because there is no double bond for the bromine to add across.
An alkene has a carbon to carbon double bond and is unsaturated. The general formula of a simple alkene with one double bond is CₙH₂ₙ. Ethene is C₂H₄. Bromine water, which is brown, loses its colour when ethene is bubbled through it, because bromine adds across the double bond. That test distinguishes ethene from ethane. Alkenes also burn, so combustion does not tell them apart. The test does.
Saturated means no carbon to carbon double bond, not "it is full of hydrogen" with no bond named. The general formulae must match the family. A candidate who gives CₙH₂ₙ for alkanes has swapped the families. The bromine water test needs the observation: brown to colourless for an alkene, no change for an alkane. "It reacts" without the colour earns less. Decolourise is the verb. The product is colourless, not clear, though clear is often tolerated. Prefer colourless.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
3 marks
(a) State the general formula of an alkane and of an alkene with one double bond. (2 marks)
(b) Write the molecular formula of ethene. (1 mark)
(a) Alkanes CₙH₂ₙ₊₂. Alkenes CₙH₂ₙ.
(b) C₂H₄.
4 marks
Describe a test to distinguish ethene from ethane, and state the result with each gas. (4 marks)
Bubble each gas through bromine water. Ethene turns the brown bromine water colourless. Ethane leaves the bromine water brown.
2 marks
Explain why ethene is described as unsaturated. (2 marks)
It contains a carbon to carbon double bond, so it can add more atoms across that bond. An alkane, with only single bonds, cannot.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Chemistry freeUse bromine water. An alkene such as ethene turns brown bromine water colourless by addition across the double bond. An alkane such as ethane leaves it brown. Both gases burn, so a flame is not the test.
The molecule contains a carbon to carbon double bond, or another multiple bond, and can add more atoms. Ethene, C₂H₄, is unsaturated. Ethane, C₂H₆, is saturated. All WASSCE Chemistry past questions