What you'll learn
Combustion of fuels and incomplete combustion sits in the organic chemistry section of AQA GCSE Chemistry and connects directly to the atmosphere unit, because almost every atmospheric pollutant you need to know originates in a combustion reaction. The topic is conceptually simple — a fuel burns in oxygen and releases energy — but the details matter enormously: whether the air supply is plentiful or limited changes the products completely, and each product has its own distinct hazard. By the end of this guide you should be able to write word and balanced symbol equations for complete and incomplete combustion, explain why incomplete combustion occurs and what it produces, describe the specific hazards of carbon monoxide and particulates, explain the origins of sulfur dioxide and oxides of nitrogen, describe the tests for the products of combustion, and evaluate hydrogen as an alternative fuel.
Key terms and definitions
Combustion — the reaction of a substance with oxygen, releasing energy; also called burning
Complete combustion — combustion in a plentiful supply of oxygen, producing carbon dioxide and water only
Incomplete combustion — combustion in a limited supply of oxygen, producing carbon monoxide and carbon in addition to or instead of carbon dioxide
Hydrocarbon — a compound containing hydrogen and carbon only
Exothermic — describing a reaction that transfers energy to the surroundings
Oxidation — in this context, the gaining of oxygen by the carbon and hydrogen in the fuel
Particulate — a small solid particle of carbon or unburnt hydrocarbon released into the air
Global dimming — a reduction in the sunlight reaching the Earth's surface, caused partly by particulates
Acid rain — rain made acidic by dissolved sulfur dioxide and oxides of nitrogen
Fuel — a substance that releases energy when burned
Core concepts
Complete combustion
When a hydrocarbon burns in a plentiful supply of oxygen, both the carbon and the hydrogen in the fuel are oxidised. The carbon becomes carbon dioxide and the hydrogen becomes water, and energy is released. The reaction is strongly exothermic, which is what makes hydrocarbons useful as fuels.
The general word equation is hydrocarbon plus oxygen giving carbon dioxide plus water.
For methane, one molecule reacts with two molecules of oxygen to give one molecule of carbon dioxide and two molecules of water. Balancing combustion equations is a standard examination task, and the reliable order is to balance carbon first, then hydrogen, then oxygen last, since oxygen appears in both products and is therefore easiest to adjust at the end.
Complete combustion is recognised in practice by a clean blue flame, which indicates efficient burning and maximum energy release.
Incomplete combustion
When the supply of oxygen is limited, there is not enough to oxidise all the carbon fully. Some carbon is only partly oxidised to carbon monoxide, and some is not oxidised at all and is released as solid carbon particles, known as soot or particulates. Water is still produced.
Incomplete combustion releases less energy per unit of fuel than complete combustion, because the fuel has not been fully oxidised. It is recognised by a yellow or orange smoky flame, and by soot deposits on surfaces above the flame.
The products may therefore include carbon dioxide, carbon monoxide, carbon and water, in proportions depending on how restricted the oxygen supply is. A question asking what incomplete combustion produces expects carbon monoxide and particulates to be named specifically.
Incomplete combustion occurs in practice when a gas appliance is poorly maintained, when a vent or flue is blocked, or when a device is used in an enclosed space.
Why carbon monoxide is so dangerous
Carbon monoxide is toxic. It binds to haemoglobin in red blood cells in place of oxygen, so the blood carries less oxygen around the body. The effects begin with headache and drowsiness and can progress to unconsciousness and death.
What makes it exceptionally hazardous is that it is colourless, odourless and tasteless, so a person cannot detect it by their senses and may become drowsy and lose consciousness without realising anything is wrong. This combination of toxicity and undetectability is the reason carbon monoxide detectors are fitted in homes, and it is the point examiners look for.
Particulates and their effects
Particulates are solid particles of carbon and unburnt hydrocarbons released into the atmosphere.
They cause global dimming, reducing the amount of sunlight reaching the Earth's surface by reflecting it back into space and by seeding cloud formation.
They also damage human health. Because the particles are very small they can be inhaled deep into the lungs, where they cause and worsen respiratory problems.
Particulates additionally deposit on buildings and surfaces as soot, causing soiling and damage.
Sulfur dioxide and oxides of nitrogen
These two pollutants arise from combustion but not from the hydrocarbon itself, and their different origins are frequently confused.
Sulfur dioxide is produced when a fuel containing sulfur impurities is burned. The sulfur present in the fuel is oxidised to sulfur dioxide. Crude oil fractions naturally contain sulfur compounds, which is why sulfur is removed from fuels at the refinery in modern practice.
Oxides of nitrogen form differently. They are not produced from the fuel at all. Inside an engine the temperature is high enough for nitrogen and oxygen from the air itself to react together, forming nitrogen monoxide and nitrogen dioxide.
Both pollutants dissolve in water in the atmosphere to produce acid rain. Acid rain damages buildings and statues made of limestone and marble, harms plants and trees, and makes lakes and rivers acidic, killing aquatic life. Both also cause respiratory problems in humans.
Summarised for the examination: sulfur dioxide comes from sulfur impurities in the fuel; oxides of nitrogen come from nitrogen and oxygen in the air reacting at high temperature.
Testing for the products
The products of combustion can be identified experimentally, and the tests are examinable.
Water is tested with anhydrous copper sulfate, which turns from white to blue, or with cobalt chloride paper, which turns from blue to pink. To confirm the liquid is pure water rather than a solution, check that it boils at 100 degrees Celsius.
Carbon dioxide is tested by bubbling the gas through limewater, which is an aqueous solution of calcium hydroxide. A positive result is that the limewater turns milky, or cloudy white.
In a typical practical, the gases from a burning fuel are drawn through a cooled U-tube, where water condenses and can be tested, and then through limewater, which turns milky if carbon dioxide is present. The order matters: the water must be collected before the gas reaches the limewater.
Hydrogen as an alternative fuel
Hydrogen burns in oxygen to produce water only, with no carbon dioxide, no carbon monoxide and no particulates. This makes it attractive as a fuel, and comparing it with hydrocarbon fuels is a standard evaluation question.
The advantages are that the only product is water, so it does not contribute to climate change at the point of use, and that it releases a large amount of energy per gram.
The disadvantages are practical and significant. Hydrogen is a gas at room temperature, so it is difficult and expensive to store and transport, requiring either very high pressure or very low temperature. It is highly flammable and forms explosive mixtures with air. Most importantly, producing hydrogen requires energy, and if that energy comes from burning fossil fuels then carbon dioxide is released elsewhere in the process.
A balanced answer acknowledges that hydrogen is clean at the point of use but that its overall environmental benefit depends on how it is produced.
Worked examples
Example 1: Balancing a combustion equation (3 marks)
Balance the equation for the complete combustion of propane, which has three carbon atoms and eight hydrogen atoms.
Balance carbon first: three carbon atoms in the fuel require three molecules of carbon dioxide. Balance hydrogen next: eight hydrogen atoms require four molecules of water, since each water molecule contains two hydrogen atoms. Now count the oxygen atoms needed on the right: three carbon dioxide molecules contain six oxygen atoms and four water molecules contain four, giving ten in total. Since each oxygen molecule supplies two atoms, five oxygen molecules are required. The balanced equation therefore has one propane and five oxygen on the left, and three carbon dioxide and four water on the right.
Example 2: Explaining a change in flame (4 marks)
A gas burner produces a clean blue flame when its air hole is open, but a yellow smoky flame when the air hole is closed. Explain this observation and state one danger.
With the air hole open there is a plentiful supply of oxygen, so complete combustion occurs. All the carbon is oxidised to carbon dioxide and all the hydrogen to water, producing a clean blue flame and releasing the maximum energy.
With the air hole closed the oxygen supply is limited, so incomplete combustion occurs. Some carbon is only partly oxidised to carbon monoxide and some is released as solid carbon particles, which glow and produce the yellow smoky flame and deposit soot.
The danger is that carbon monoxide is toxic and is colourless and odourless, so a person in the room could be affected without detecting its presence.
Example 3: Distinguishing pollutant origins (3 marks)
Explain why sulfur dioxide and oxides of nitrogen are both released by a car engine, even though petrol contains neither sulfur dioxide nor nitrogen compounds.
Sulfur dioxide is formed because petrol may contain small quantities of sulfur as an impurity from the crude oil it was refined from. When the fuel burns, this sulfur is oxidised to sulfur dioxide.
Oxides of nitrogen are not formed from the fuel at all. The temperature inside the engine is high enough to make nitrogen and oxygen from the air react together, producing nitrogen monoxide and nitrogen dioxide. Both pollutants then dissolve in atmospheric water to form acid rain.
Common mistakes and how to avoid them
The most frequent error is giving carbon dioxide as the distinctive product of incomplete combustion. Incomplete combustion is defined by carbon monoxide and particulates; carbon dioxide may still be present but it is not what identifies the process.
Students often state that carbon monoxide is dangerous simply because it is poisonous, without mentioning that it cannot be detected by smell or sight. The undetectability is usually worth a separate mark.
A common confusion is claiming that oxides of nitrogen come from nitrogen in the fuel. They come from nitrogen in the air, reacting at high temperature.
In balancing questions, many candidates change a formula rather than a balancing number. Only the large numbers in front may be altered.
Finally, answers about limewater frequently say it turns clear. It turns milky or cloudy white; limewater is already clear in the sense of transparent.
Exam technique for "Combustion of fuels and incomplete combustion"
Balance combustion equations in the fixed order carbon, hydrogen, oxygen. Leaving oxygen until last works because it appears in both products and can absorb whatever remains.
When a question describes a flame colour or a soot deposit, identify the type of combustion first and then reason from it. The flame colour is the evidence; the oxygen supply is the explanation.
Always pair a pollutant with its effect. Naming carbon monoxide earns less than naming it and stating that it is toxic and undetectable.
For evaluation questions on hydrogen, give at least two advantages and two disadvantages and finish with a judgement that distinguishes emissions at the point of use from emissions across the whole production process.
Quick revision summary
Complete combustion occurs in plentiful oxygen and produces carbon dioxide and water only, releasing maximum energy with a clean blue flame. Incomplete combustion occurs in limited oxygen and produces carbon monoxide and solid carbon particulates as well as water, releasing less energy with a yellow smoky flame. Carbon monoxide is toxic because it reduces the blood's capacity to carry oxygen, and is especially dangerous because it is colourless and odourless. Particulates cause global dimming and respiratory harm. Sulfur dioxide comes from sulfur impurities in the fuel, while oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperatures inside an engine; both dissolve to give acid rain, damaging buildings, plants and lakes, and both cause respiratory problems. Water is identified with anhydrous copper sulfate turning white to blue or cobalt chloride paper turning blue to pink, and carbon dioxide by limewater turning milky. Hydrogen produces only water when burned but is difficult to store, highly flammable, and its overall benefit depends on how it is manufactured.