Organic chemistry & analysis
Crude oil and hydrocarbons, alkanes, alkenes and polymers, and the tests that identify common gases and ions.
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Organic chemistry & analysis, explained point by point
Everything the GCSE specification expects you to be able to do, and how to actually do it - the same lesson a signed-in student studies from.
Describe crude oil, fractional distillation and hydrocarbons
Crude oil is a mixture of hydrocarbons - compounds of only hydrogen and carbon - formed over millions of years. Fractional distillation separates it: the oil is heated and the vapours rise up a column, condensing at different heights by boiling point. Shorter molecules come off the top as fuels like petrol; longer ones settle lower as bitumen.
Describe alkanes and alkenes
Alkanes are saturated hydrocarbons with only single bonds, following the formula CₙH₂ₙ₊₂ - methane (CH₄) and ethane are the first two. Alkenes contain a carbon–carbon double bond, making them more reactive, and turn bromine water from orange to colourless. That colour change is the test that tells the two families apart.
Explain combustion and cracking
Complete combustion of a hydrocarbon in plenty of oxygen produces carbon dioxide and water and releases energy - how fuels warm us. Cracking breaks long, less useful hydrocarbon chains into shorter ones using heat and a catalyst, giving more petrol and useful alkenes for making plastics. It balances supply with what people actually want.
Describe polymers and how they form
Polymers are very long molecules built from many small repeating units called monomers. In addition polymerisation, alkene monomers with their double bonds join up with nothing left over - many ethene molecules make poly(ethene). Polymers make plastics, but because many do not break down easily, disposing of them is an environmental challenge.
Carry out tests for hydrogen, oxygen, carbon dioxide and chlorine
Each common gas has a simple test. Hydrogen gives a squeaky pop with a lit splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater cloudy. Chlorine bleaches damp litmus paper white. Knowing the result AND the observation lets you identify an unknown gas with confidence.
Explain why pure substances have sharp melting points
A pure substance melts and boils at a fixed, sharp temperature, while a mixture melts over a range. So checking a melting point is a quick test of purity. Chromatography separates a mixture of dyes and gives an Rf value - the distance a spot travels compared with the solvent - helping identify what is present.
Organic chemistry & analysis key terms
The words the specification and the mark schemes use, each defined the way an examiner wants it.
- Hydrocarbon
- A compound made of hydrogen and carbon atoms only.
- Alkane
- A saturated hydrocarbon with only single bonds, general formula CnH2n+2. The first four are methane, ethane, propane and butane.
- Alkene
- A hydrocarbon with a carbon-carbon double bond, making it more reactive than an alkane. Alkenes turn bromine water colourless.
- Fractional distillation
- Separating crude oil into fractions of similar boiling points, from gases at the top of the column to bitumen at the bottom.
- Cracking
- Breaking down long hydrocarbon molecules into smaller, more useful ones, producing shorter alkanes and alkenes.
- Pure substance
- In chemistry, a single element or compound not mixed with anything else. A pure substance melts and boils at specific, sharp temperatures.
- Formulation
- A useful mixture made to a precise recipe, such as a paint, a medicine or a fuel.
- Chromatography
- A technique for separating the substances in a mixture as they move through a stationary phase at different speeds. Rf = distance moved by the substance ÷ distance moved by the solvent.
Practice
Try a Organic chemistry & analysis question
A GCSE-style original question from this topic. Have a go before you open the working - deciding on an answer first is what makes the working stick.
Propane is an alkane with three carbon atoms. Using the general formula CnH2n+2, how many hydrogen atoms does one molecule of propane have?
- 3
- 6
- 9
- 8
Show the answer and the working
Answer: 8
For an alkane, hydrogen atoms = 2n + 2, and with n = 3 this gives 2 × 3 + 2 = 8.
- Propane is an alkane, so use CnH2n+2.
- It has n = 3 carbon atoms.
- Hydrogen atoms = 2 × 3 + 2 = 8.