5.3.2 Combustion

What is Combustion?

It means burning .

A substance burns by reacting with oxygen from air, and gives out heat and light .

Examples

• Burning of wood in a fire.
• Burning of paper with a matchstick.
• Burning of LPG on a gas stove.
• Burning of a candle.
• Burning of magnesium ribbon in air.

Key features of combustion

• The substance reacts with oxygen from the air.
• Heat is produced.
• Often, light is also produced.
• A new substance is formed (so it is always a chemical change).

Combustible substances

• The substance that burns is called a combustible substance .
• It is also called a fuel .
• Common combustible substances:
  • Wood.
  • Paper.
  • Cotton.
  • Kerosene.
  • LPG (cooking gas).
  • Coal.

Example — Burning a Magnesium Ribbon

This is one of the clearest examples of combustion.

What you observe

• A magnesium ribbon is burnt in air.
• It produces a brilliant white flame .
• A white powder is left behind.
• This white powder is a new substance — magnesium oxide .
• Heat and light are also given out.

Why this is a chemical change

• A new substance (magnesium oxide) has been formed.
• So this burning is a chemical change — specifically, it is combustion.

(ribbon)      (from air)      (white powder)

The Three Requirements for Combustion (The Fire Triangle)

For burning to happen, three things must be present together.

The three requirements

• (i) Fuel
  • This is the combustible substance — the thing that burns.
  • Examples: wood, paper, kerosene, LPG.
• (ii) Oxygen
  • It is taken from the surrounding air.
  • Without oxygen, the fuel cannot burn.
• (iii) Heat
  • Enough heat to raise the fuel to its ignition temperature .
  • Below this temperature, the fuel will not catch fire.

What happens if any one is missing

• Take away the fuel — the fire dies out.
• Take away the oxygen — the fire goes out (this is how a fire blanket works).
• Take away the heat — the fire never starts.

What is Ignition Temperature?

It means the lowest temperature at which a substance catches fire.

Examples

• Paper has a low ignition temperature, so a matchstick lights it instantly.
• Wood has a higher ignition temperature, so it takes longer to catch fire.
• Coal has an even higher ignition temperature.

Key idea about ignition temperature

• Below this temperature, the substance will not catch fire on its own.
• Even paper will sit safely in air at room temperature.
• A burning matchstick is hotter than the ignition temperature of paper.
• So paper catches fire immediately when touched by a matchstick.

👀 Activity 5.5 — Is Oxygen Needed for Combustion?

Activity 5.5 — Let Us Investigate

⚠️ Caution — Perform this activity under the supervision of a teacher or an adult.

Materials needed

• Two identical candles .
• Two petri dishes (or any flat plates).
• A glass tumbler .
• Optional: a small amount of lime water .

Procedure

• Place the two candles on separate petri dishes and light them both.
• Cover one of the candles with the inverted glass tumbler.
• Leave the other candle uncovered.
• Observe what happens to both flames over the next minute.

Observation

• The uncovered candle keeps burning .
• The covered candle stops burning after a short time.
• If you add a few drops of lime water inside the inverted tumbler afterwards, it turns milky .

Explanation

• The covered candle uses up the oxygen in the trapped air.
• Without continuous oxygen supply, combustion stops — the flame goes out.
• The lime water turning milky shows that carbon dioxide was produced inside — from the carbon in the wax reacting with the oxygen in the trapped air.
• Conclusion: Oxygen is required for combustion.

→ Cut off the oxygen, and even an actively burning fire goes out — this is exactly how a fire blanket works.

👀 Activity 5.6 — Heat is Also Needed: Sunrays Through a Magnifying Glass

Activity 5.6 — Let Us Investigate

⚠️ Caution — Perform this activity under the supervision of your teacher or an adult.

Materials needed

• A piece of paper .
• A pair of tongs .
• A lighted matchstick .
• A magnifying glass .
• A sunny day.

Procedure

• Step 1: Hold a piece of paper with the tongs and bring a lighted matchstick close to it. The paper catches fire immediately.
• Step 2: Take another piece of paper. Use the magnifying glass to focus sunrays into the smallest, brightest spot on the paper.
• Hold the focus there for some time and observe.

Observation

• Step 1: The matchstick fire instantly ignites the paper.
• Step 2: After holding the focused sunrays on one spot for a while, the paper starts emitting smoke and then catches fire — even without any flame nearby .

Explanation

• Focusing sunrays heats the paper at one spot.
• The temperature of that spot rises with time.
• When the temperature crosses the paper's ignition temperature , it catches fire on its own.
• The matchstick was already much hotter than paper's ignition temperature, so it ignited the paper instantly.
• Conclusion: A combustible substance + oxygen is not enough — you also need heat to reach the ignition temperature.

→ Combustion needs all three: fuel + oxygen + heat to ignition temperature .

🌟 Fascinating Facts — Nature's Wonders: Bioluminescence

• You may have seen tiny insects emitting light in a garden or field on a late evening.
• These are fireflies .
• The light is produced by a chemical change inside the insect's body.
• Light produced by a chemical change in a living organism — without heat — is called bioluminescence .
• So this is a chemical change that gives off light only , no heat — very different from combustion.