Burning Fuels

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.11 — A fuel as a substance that releases heat energy when burned
• 4.12 — The products of complete and incomplete combustion of hydrocarbons
• 4.13 — Why carbon monoxide is poisonous

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A FUEL is simply a substance that releases heat energy when it is burned. That is the whole definition, and it is worth noticing how little it demands: nothing about where the substance came from, nothing about how much energy, nothing about how cleanly it burns. Hydrogen is a fuel and so is coal, and so is every fraction from the middle of the fractionating column.

Burning a hydrocarbon means reacting it with oxygen, and the products depend on one thing above all others: whether there is enough oxygen to go round. The specification treats this as two named cases, and the difference between them is worth more marks than any other idea on this page.

Complete combustion — plenty of oxygen

1. Every carbon atom in the hydrocarbon finds enough oxygen to become CARBON DIOXIDE.
2. Every hydrogen atom finds enough oxygen to become WATER.
3. So the only products are carbon dioxide and water. Nothing else.
4. The flame is clean and blue, because there is no glowing solid carbon in it.
5. The maximum possible energy is released, because every atom has been oxidised as far as it can go.
6. For methane: CH₄ + 2O₂ → CO₂ + 2H₂O.

Incomplete combustion — not enough oxygen

1. The hydrogen atoms are oxidised first, so WATER is still formed in every case.
2. There is now not enough oxygen left for every carbon to reach carbon dioxide.
3. Some carbon atoms get only one oxygen each, forming CARBON MONOXIDE, CO — a colourless, odourless, poisonous gas.
4. Where the shortage is severe, some carbon atoms get no oxygen at all and are released as solid carbon — SOOT.
5. The flame is yellow or orange and smoky, because tiny particles of hot solid carbon in it glow.
6. LESS energy is released than in complete combustion, because the carbon has not been oxidised as far as it could be.

Think of it like handing out coats at a cloakroom that has run short

If there are enough coats, everyone leaves fully dressed. If there are not, the people at the front still get theirs, some get only one sleeve, and the last few get nothing at all. Nobody is turned away — they simply leave less well equipped. Oxygen atoms are the coats. When there is plenty, every carbon gets two and becomes carbon dioxide. When supply runs short, some carbons get one and become carbon monoxide, and the unlucky ones get none and leave as soot. The fuel still burns; it simply burns less completely.

This is not a laboratory curiosity. A gas boiler with a blocked flue, a portable heater used in a sealed room, a barbecue brought indoors — all of these restrict the oxygen supply and shift a flame from complete towards incomplete combustion. The soot is visible and the reduced heat output is noticeable. The carbon monoxide is neither.

Why carbon monoxide is poisonous — statement 4.13

1. Oxygen is carried around the body attached to HAEMOGLOBIN, the red pigment inside red blood cells.
2. Carbon monoxide binds to haemoglobin at the same site that oxygen uses.
3. It binds there FAR more strongly than oxygen does, and is not readily released again.
4. Haemoglobin carrying carbon monoxide therefore cannot pick up oxygen at the lungs.
5. So less oxygen reaches the body's cells, and respiration in those cells slows or stops.
6. The danger is made worse by the gas being colourless and odourless, so a person breathing it has no warning at all — drowsiness and headache come first, and they can be mistaken for tiredness.

Notice that carbon monoxide does not damage the lungs or attack the blood. It does something more economical: it occupies the seats. The oxygen arriving at the lungs is perfectly good, and there is simply nowhere for it to sit.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Fuel

A substance that releases heat energy when it is burned.

Complete against incomplete combustion

CH₄ + 2O₂ → CO₂ + 2H₂O

Conditions: Complete combustion of methane in plenty of oxygen. Products are carbon dioxide and water only.

2CH₄ + 3O₂ → 2CO + 4H₂O

Conditions: Incomplete combustion of methane in limited oxygen, giving carbon monoxide instead of carbon dioxide.

CH₄ + O₂ → C + 2H₂O

Conditions: Severely restricted oxygen: the carbon is released as solid soot, which is what makes the flame yellow.

The vocabulary this page is marked on

Complete combustion

Burning in plenty of oxygen, giving carbon dioxide and water only.

Incomplete combustion

Burning in a limited supply of oxygen, giving carbon monoxide and/or carbon as well as water.

Soot

Solid carbon released when combustion is very incomplete. It makes a flame yellow and smoky.

Haemoglobin

The red pigment in red blood cells that carries oxygen around the body.

Model answer [4 marks]

Explain why carbon monoxide is poisonous. [4]

Oxygen is normally carried around the body attached to haemoglobin in the red blood cells. Carbon monoxide binds to haemoglobin at the same site as oxygen, and it binds far more strongly than oxygen does and is not readily released. Haemoglobin that has taken up carbon monoxide can therefore no longer pick up oxygen at the lungs, so less oxygen is delivered to the body's cells and respiration in them is reduced. The danger is increased because carbon monoxide is colourless and odourless, so someone breathing it receives no warning.

Model answer [4 marks]

A gas heater burns with a yellow, smoky flame instead of a blue one. Explain what this indicates and why it is dangerous. [4]

A yellow, smoky flame shows incomplete combustion, caused by an insufficient supply of oxygen. The yellow colour comes from particles of hot solid carbon glowing in the flame, and the smoke is soot. Because the carbon is not being fully oxidised, less energy is released than the heater should give. The serious hazard is that incomplete combustion also produces carbon monoxide, which binds to haemoglobin and prevents it from carrying oxygen; since carbon monoxide is colourless and odourless, the occupants of the room would have no warning that it was present.

Writing a balanced combustion equation

1. Write the hydrocarbon and O₂ on the left, and decide what the carbon becomes: CO₂ if complete, CO or C if incomplete.
2. Balance the CARBON first: one carbon in the molecule of fuel gives one carbon-containing product molecule.
3. Balance the HYDROGEN next: every two hydrogen atoms give one molecule of water.
4. Count the oxygen atoms now needed on the right, and divide by two to find how many O₂ molecules are required.
5. If that number is a fraction, double every coefficient in the equation so all of them are whole numbers.

What tells you a flame is burning incompletely

• The flame is yellow or orange rather than blue
• The flame is smoky, and soot is deposited on anything held in it
• Less heat is produced than expected
• Carbon monoxide is being formed, though nothing about it can be seen or smelled

Not this: Carbon monoxide is poisonous because it damages the lungs or destroys the blood.

This: It does neither. It occupies the site on haemoglobin that oxygen would use, and holds it far more tightly than oxygen does, so oxygen cannot be picked up and carried. The lungs and the blood are undamaged; the oxygen simply has nowhere to go.

Mark-losing trap. Water is produced by BOTH types of combustion. Only the carbon-containing product changes.

Mark-losing trap. Incomplete combustion releases LESS energy, not more. The carbon has not been oxidised as far as it could be.

Mark-losing trap. Say carbon monoxide binds "far more strongly" than oxygen, not "permanently" — the binding is strong, not irreversible.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What are the only products formed when a hydrocarbon undergoes complete combustion?
2. Grade 7 · Explain [2 marks] — A Bunsen burner flame turns from blue to yellow when the air hole is closed. Explain why the colour changes.
3. Grade 8 · Compare [3 marks] — Give TWO differences, other than flame colour, between complete and incomplete combustion of a hydrocarbon.
4. Grade 9 · Explain [4 marks] — Select every statement that belongs in a full explanation of why carbon monoxide from a faulty heater can kill someone in a closed room.
5. 9+ · Write [5 marks] — Write a balanced equation for the incomplete combustion of propane, C₃H₈, in which every carbon atom forms carbon monoxide. Give the number that should go in front of the O₂.

The people behind this science

Two ways into the same idea — the one who established what burning actually is, and the one who worked out what the gas does once it is in the blood. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antoine Lavoisier — the one who established what burning actually is

Everything on this page depends on knowing that burning is a substance COMBINING with oxygen, and that is Lavoisier's result. Before him, chemists explained burning by phlogiston — a substance supposedly released by anything that burns — and the theory was wrecked by a single measurement Lavoisier insisted on making: burning a metal in a sealed vessel makes it heavier, not lighter, and the surrounding air loses exactly the mass the metal gains. Once burning is combination with oxygen, the difference between complete and incomplete combustion becomes a question about how much oxygen there is, which is exactly how this lesson treats it.

• “What convinced you that burning was combination and not release?”
• “Why did weighing everything in a sealed vessel matter so much?”
• “What did phlogiston explain that your account had to explain too?”
• “Does a flame need air, or does it need something in the air?”
• “How would you show that burning a fuel produces water?”

Claude Bernard — the one who worked out what the gas does once it is in the blood

Statement 4.13 asks why carbon monoxide is poisonous, and the answer given on this page is Bernard's discovery. In the 1850s he showed that the poisoning is not damage to the lungs or to the tissues but a substitution in the blood itself: carbon monoxide takes the place oxygen would occupy and holds it, so the blood stays a bright red while the animal suffocates. That result is the reason the modern explanation is about haemoglobin rather than about the lungs, and Bernard is the right person to ask how you demonstrate a mechanism that is invisible from the outside.

• “How did you show the poisoning happens in the blood itself?”
• “Why does blood stay bright red when carbon monoxide is present?”
• “What made you look at the blood rather than at the lungs?”
• “How can you tell a substance blocks something rather than destroying it?”
• “What does poisoning teach you about how the body normally works?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Robert Bunsen in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Crude Oil And Its Fractions
• Next lesson: Pollutants And Acid Rain
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• Cracking — Breaking long molecules nobody wants into short ones everybody does — and the reason a refinery has to.
• All of Chemistry · Incandio Science