CombustionIB MYP Sciences: Revision notes
Section 1
What is combustion?
Combustion is burning: a fuel reacts with oxygen and releases energy as heat and light. Combustion is an exothermic reaction because energy is transferred to the surroundings, so the temperature rises.
fuel + oxygen → oxides + energy
A fuel is a substance that releases useful energy when it burns, such as methane, petrol, coal or wood. Most fuels contain carbon and hydrogen, so they burn to make carbon dioxide and water.
Section 2
Complete combustion
When there is plentiful oxygen, a fuel that contains carbon and hydrogen burns completely. The carbon becomes carbon dioxide and the hydrogen becomes water.
methane + oxygen → carbon dioxide + water
CH₄ + 2O₂ → CO₂ + 2H₂O
The flame is blue and very little soot is made. Complete combustion releases the most energy from the fuel.
Section 3
Incomplete combustion
If the supply of oxygen is limited, a fuel burns incompletely. Some carbon is not fully oxidised, so the products can include carbon monoxide (CO) and soot (carbon, C), as well as water.
methane + oxygen → carbon monoxide + water
2CH₄ + 3O₂ → 2CO + 4H₂O
The flame is yellow because of glowing carbon particles, and a black deposit forms on cooking pots. Incomplete combustion releases less energy than complete combustion.
Incomplete combustion does not mean there is no oxygen. It means there is not enough oxygen for the fuel to burn completely.
Section 4
The fire triangle
Combustion needs three things at the same time: fuel, oxygen and heat. This is the fire triangle. Remove any one side and the fire goes out.
- remove fuel: turn off the gas
- remove oxygen: cover with a fire blanket or a lid
- remove heat: cool with water (not on oil or electrical fires)
This is how a candle goes out under a jar: the oxygen is used up.
Section 5
Carbon monoxide hazards
Carbon monoxide is a toxic gas. It stops the blood carrying oxygen around the body, which causes headaches, drowsiness and, in high amounts, death.
It is especially dangerous because it is colourless and has no smell. It forms when gas heaters, boilers, stoves or car engines burn fuel with too little oxygen.
Safety measures:
- make sure rooms have good ventilation
- have heaters and boilers serviced regularly
- fit a carbon monoxide alarm
If asked why carbon monoxide is dangerous, say it is toxic AND that it is colourless and odourless, so people cannot detect it.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Combustion
- A chef in Lagos cooks on a gas hob that burns natural gas, which is mainly methane. When the air holes at the base of the burner are open, the flame is blue and the pan stays clean. When the air holes are closed, the flame becomes yellow and the base of the pan is coated in a black deposit.Explain why the flame is yellow and produces a black deposit when the air holes are closed.2 marks
- A family in Montreal runs a gas heater in a small bedroom during a cold winter. The windows are sealed to keep the heat in. During the night, a carbon monoxide alarm in the room sounds loudly.Suggest two ways in which the family could reduce the risk of carbon monoxide poisoning.2 marks
- Alana, a student in Wellington, investigates how long a candle burns under glass jars of different volumes. She lights three identical candles, covers each with an upturned jar and times how long it takes for each flame to go out. The 250 cm³ jar gave 8 seconds, the 500 cm³ jar gave 17 seconds and the 1000 cm³ jar gave 33 seconds. She tested each jar once.Describe the trend in Alana's results and explain it using the fire triangle.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).