Incomplete Combustion: What It Is, Why It Happens, and How to Answer Exam Questions on It

What is incomplete combustion?

Combustion is a chemical reaction where a fuel reacts with oxygen and releases energy. When a hydrocarbon fuel (like methane, propane, or octane) burns in plenty of oxygen, every carbon atom is fully oxidised to carbon dioxide (CO₂) and every hydrogen atom forms water (H₂O). This is called complete combustion.

But what happens when there is not enough oxygen for every carbon atom to become CO₂?

The carbon atoms compete for the limited oxygen. Some carbon atoms get fully oxidised to CO₂, but others only partially oxidise to carbon monoxide (CO), and some do not oxidise at all and are released as carbon (C), which we see as soot or black smoke.

This is incomplete combustion. The fuel still burns, it still releases energy, but the products are different and the reaction releases less energy than complete combustion.

Complete vs incomplete combustion

Think of it this way: complete combustion is clean and efficient, while incomplete combustion is dirty and wasteful. Both produce water, but incomplete combustion also produces harmful by-products.

Products of incomplete combustion

There are three possible carbon-containing products, depending on how limited the oxygen supply is:

1. Carbon dioxide (CO₂) - some carbon atoms still get fully oxidised, even in a limited supply.
2. Carbon monoxide (CO) - carbon atoms that are only partially oxidised. This is the dangerous one.
3. Carbon / soot (C) - carbon atoms that are not oxidised at all. This is what makes the flame yellow and leaves black deposits.

Water (H₂O) is always produced, regardless of whether combustion is complete or incomplete. The hydrogen atoms always have enough oxygen to form water.

Balanced equations for incomplete combustion

Unlike complete combustion, there is no single "correct" equation for incomplete combustion because the products depend on how limited the oxygen is. The exam will usually tell you which products to include. Here are the most common examples.

Methane (CH₄)

Complete combustion:

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion (producing CO):

2CH₄ + 3O₂ → 2CO + 4H₂O

Incomplete combustion (producing C):

CH₄ + O₂ → C + 2H₂O

Propane (C₃H₈)

Complete combustion:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Incomplete combustion (producing CO):

2C₃H₈ + 7O₂ → 6CO + 8H₂O

Notice how the incomplete combustion equations use fewer moles of oxygen. That is the key sign: less O₂ in = incomplete combustion out.

Need to practise balancing? Try our Balancing Act tool for instant feedback on any equation.

Why is incomplete combustion dangerous?

The main danger is carbon monoxide (CO). Here is why it is so harmful:

• It is colourless and odourless, so you cannot detect it without a sensor.
• It binds to haemoglobin in red blood cells. It actually binds more strongly than oxygen does, forming carboxyhaemoglobin.
• This reduces the oxygen-carrying capacity of the blood. Organs and tissues do not get the oxygen they need.
• Symptoms include headaches, dizziness, nausea, and confusion. In severe cases, it causes unconsciousness and death.

This is why gas appliances (boilers, cookers, heaters) must be properly maintained and rooms must be well-ventilated. A faulty gas boiler in a poorly ventilated room is one of the most common sources of carbon monoxide poisoning.

Carbon monoxide detectors are inexpensive and can save lives. They are now required by law in many homes.

The other product, soot (carbon), contributes to air pollution. Soot particles are small enough to be inhaled and can cause respiratory problems. Soot also blackens buildings and contributes to global dimming, where particles in the atmosphere reduce sunlight reaching the Earth's surface.

Worked examples

The Bunsen burner connection

The Bunsen burner is the perfect demonstration of complete vs incomplete combustion. It is also a very common exam question.

• Air hole open: oxygen mixes with the gas before it burns. The flame is blue (complete combustion). This produces more heat and is used for heating in experiments.
• Air hole closed: less oxygen reaches the gas. The flame is yellow (incomplete combustion). This is called the safety flame because it is more visible, even though it is less efficient.

The safety flame produces less heat but is easier to see, which reduces the risk of accidentally putting something into the flame. This is why you always light the Bunsen burner with the air hole closed first, then open it when you need a hotter flame.

Exam tips for combustion questions

For more detail on combustion, fuels, and hydrocarbons, see our full AQA Topic 9: Chemistry of the Atmosphere revision notes.

Frequently Asked Questions

What is incomplete combustion?

Incomplete combustion happens when a fuel burns in a limited supply of oxygen. Instead of producing only carbon dioxide and water, it produces carbon monoxide (CO), carbon (soot), or a mixture of both, along with water. It releases less energy than complete combustion.

Why is incomplete combustion dangerous?

It produces carbon monoxide (CO), which is a colourless and odourless toxic gas. Carbon monoxide binds to haemoglobin in red blood cells more strongly than oxygen does, reducing the blood's ability to carry oxygen. This leads to headaches, dizziness, and in severe cases, death.

What is the difference between complete and incomplete combustion?

Complete combustion occurs in plenty of oxygen and produces only CO₂ and H₂O. Incomplete combustion occurs in limited oxygen and produces CO, C, or both, along with water. Complete combustion gives a blue flame and releases more energy. Incomplete combustion gives a yellow flame and releases less energy.

What colour flame indicates incomplete combustion?

A yellow or orange flame indicates incomplete combustion. The yellow colour comes from tiny glowing particles of carbon (soot) in the flame. A blue flame indicates complete combustion, where all carbon is fully oxidised to CO₂. The best example of this is a Bunsen burner: open air hole gives blue, closed air hole gives yellow.