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Lesson · Grade 10 · Science

Combustion: where does the fuel go?

  • Grade 10
  • Science
  • 1 min 52 s
  • British Columbia
  • BC10-SC-21

A methane molecule and two oxygen molecules break apart in a blue gas flame, and the same atoms join up again as one carbon dioxide and two water molecules, giving out heat and light. The names and formulas under the molecules become the equation CH4 + 2O2 → CO2 + 2H2O, the atoms are counted on both sides (1 carbon, 4 hydrogen, 4 oxygen) to show nothing disappears, and a final card shows that with too little oxygen some carbon becomes carbon monoxide or soot, which is why homes need carbon monoxide alarms.

Concept: Combustion: fuel and oxygen make carbon dioxide and water

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In one sentence

The question this lesson answers

When fuel burns, it seems to vanish. So where does it go?

Burning is called combustion: a fuel reacting with oxygen.

Transcript

What the lesson says

When fuel burns, it seems to vanish. So where does it go?

Burning is called combustion: a fuel reacting with oxygen.

Natural gas is mostly methane, a hydrocarbon: a compound of only hydrogen and carbon.

It reacts with two oxygen molecules from the air.

In the flame, the bonds that hold these atoms together break.

The atoms then join up in new ways: carbon bonds to two oxygens, making carbon dioxide.

The hydrogens bond to the other two oxygens, making two molecules of water.

Making the new bonds releases more energy than breaking the old ones took, giving out heat and light.

The starting substances are the reactants, and the new substances are the products.

In symbols, one methane and two oxygen molecules give one carbon dioxide and two water molecules.

Count the atoms on each side: one carbon, four hydrogen and four oxygen, before and after.

No atoms are made or destroyed, only rearranged, so the equation balances.

So the fuel does not disappear: its atoms leave as invisible gases, carbon dioxide and water vapour.

With too little oxygen, combustion is incomplete: some carbon becomes carbon monoxide or soot.

Carbon monoxide is a colourless, odourless poison, so homes need carbon monoxide alarms.

So burning rearranges atoms: with enough oxygen, a hydrocarbon and oxygen become carbon dioxide and water.

Curriculum

Where this fits

British Columbia, Grade 10

Science 10, Chemistry: rearrangement of atoms in chemical reactionsBC10-SC-21

Previous: Single and Double Replacement ReactionsNext: Acids and bases: the pH scale

The outcomes and strands follow the British Columbia curriculum.

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