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What is Combustion?
It is a chemical reaction where substances react with oxygen and produce heat and light .
- Combustible substances can burn when they reach ignition temperature .
- Three requirements needed for combustion to occur successfully.
- Heat and light are produced during combustion reactions .
- Oxygen supply is essential for combustion to continue.
Activity 5.5: Let us investigate
Place two identical candles on two separate petri dishes and light them. Cover one of these with a glass tumbler (Fig. 5.6).
What happens to the candle flames in the two cases?
Observations:
| Setup | Observation |
|---|---|
| Candle (a): burning freely — not covered | Continues to burn normally throughout |
| Candle (b): covered with a glass tumbler | Flame gets extinguished after some time |
The candle covered by the glass tumbler does not have a continuous supply of air, so the flame gets extinguished. The component of air that supports combustion is oxygen .
You can confirm the presence of carbon dioxide gas inside the tumbler by adding a small amount of lime water in the petri dish — it turns milky. This CO₂ was formed from the carbon in wax reacting with oxygen from air. Therefore, oxygen is required for combustion .
🏫 Science and Society
If a person's clothes catch fire, what is the best way to extinguish the fire?
Wrap a blanket or cloth around the person. This cuts off the supply of air , and the fire gets extinguished.
Caution — Synthetic blanket or cloth should never be used to put out a fire, as these can melt and stick to the skin .
✨ Fascinating Facts
Nature's wonders: You might have seen some insects emitting light in a garden or a field in late evenings. These insects are called fireflies , and their light is produced by a chemical change .
This type of light production ( without heat ) in living organisms is called bioluminescence .
Activity 5.6: Let us investigate
What you need:
- Two pieces of paper
- Pair of tongs
- Lighted matchstick
- Magnifying glass
- Sunlight
What to do:
- Step 1: Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it
- Step 2: Take another piece of paper. Using a magnifying glass, focus the sunrays to make the smallest and brightest spot on the paper
- Step 3: Hold it there for some time and observe
Observations
What you see:
- Paper with matchstick catches fire immediately
- Paper with focused sunlight starts to emit smoke first
- Both papers eventually catch fire and burn
- Focused sunlight makes paper very hot before burning
Why this happens:
We have studied that
substances need ignition temperature
to start burning. This is why
both methods work
.
- Matchstick flame provides direct heat above ignition temperature
- Focused sunlight gradually heats paper to ignition temperature
- Minimum temperature needed is called ignition temperature
Question (Page Section 5.3.2): What are the three requirements for combustion to occur?
Answer:
We have studied that
combustion needs specific conditions
to happen. They are:
- Combustible substance (fuel like wood, paper)
- Oxygen (from air supply)
- Heat (to reach ignition temperature)
Section Summary: Some Other Processes Involving Chemical Changes
