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Science — Third Preparatory

Science — Third Preparatory

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Science — Third Preparatory 2026/2027

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Unit 1 - Force, Motion, Acceleration and Physical Quantities

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Text lesson

Unit 1 — Chemical Reactions and Their Environmental Impacts

Science — Third Preparatory

Unit 1 — Chemical Reactions and Their Environmental Impacts

2.6 Complete and incomplete combustion

2.6 Complete and incomplete combustion

  • Activity with a Bunsen burner.
Comparison aspectMain ideaKeywords
•Put water in a test tubewatertesttube
First open the air valve fully and light the burnerthe flame is blueflameblue
•
  • Heat the tube for a minute
  • its bottom remains clean
Heattubeminutebottom
Then close the air valve partiallythe flame turns yellowflameturnsyellow
Heat the tube againa black deposit (soot) forms on its bottomblackdepositsootforms
Key points
Blue flameplenty of oxygen entering through the air inlet; the gas burns completely.
Yellow flameair inlet narrow, little oxygen; the gas burns incompletely and unburned carbon (soot) is formed.
  • Cumulative knowledge. Bunsen burners and home gas stoves use hydrocarbons, compounds of hydrogen and carbon only.
  • Natural gas is mostly methane (CH₄).
  • Butagas (cooking gas) is mostly propane and butane.

Complete combustion of a hydrocarbon

  1. With plenty of oxygen
  2. the products are carbon dioxide and water vapour
  3. with a large amount of heat:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Balancing check→carbon 1 = 1; hydrogen 4 = 4; oxygen 4 = 2 + 2 = 4. Two oxygen molecules are required for each methane molecule.

Incomplete combustion of a hydrocarbon

  • With limited oxygen, the products depend on how much oxygen is available.
  • Producing carbon monoxide (CO):

CH₄ + 3O₂ → 2CO + 4H₂O + heat

(C→2 = 2; H: 8 = 8; O: 6 = 2 + 4 = 6.)
  1. Producing soot (carbon
  2. C):

CH₄ + O₂ → C + 2H₂O + heat

Comparison aspectMain ideaKeywords
(C
  • 1 = 1
  • H: 4 = 4
  • O: 2 = 2.)
Extra
  • with even less oxygen
  • a mixture of CO and C forms
evenlessoxygenmixture
Important note
  • burning the same mass of fuel incompletely releases less heat than burning it completely
  • so incomplete combustion is wasteful as well as polluting
burningsamemassfuel

Worked example 1: oxygen needed

  • How many oxygen molecules are needed to burn two methane molecules in each case?
Key points
Complete (to CO₂)2CH₄ + 4O₂ → 2CO₂ + 4H₂O : 4 O₂ molecules.
Incomplete to CO2CH₄ + 3O₂ → 2CO + 4H₂O : 3 O₂ molecules.
Incomplete to carbon2CH₄ + 2O₂ → 2C + 4H₂O : 2 O₂ molecules.
Conclusionthe less oxygen available, the more carbon is left in a dangerous or useless form.

Worked example 2: 4 methane molecules

  1. Complete combustion of 4 methane molecules requires 4 × 2 = 8 oxygen molecules
  2. forming 4 CO₂ and 8 H₂O.
  • A blue flame on a gas stove means complete combustion it gives carbon dioxide, water vapour and a lot of heat, so cooking is quick and less gas is used.
  • A yellow or orange flame means incomplete combustion because of a lack of oxygen: it produces toxic carbon monoxide and black soot that blackens pots, and heating is less efficient.
  • The solution is to clean the burner openings or adjust the air intake.
  • Never run a gas heater in a closed room.
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