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NCERT Solutions Curiosity Chapter 5 .3.2 Combustion · Science and Society · Is the Presence of Air Enough for Combustion? — In-text Questions

Book page 635 Updated on2026-09-05

Q1.
We observe that the candle that was not covered (Fig. 5.6a) continues to burn, whereas the candle that was covered with a glass tumbler (Fig. 5.6b) stops burning after some time. Why does this happen?
Answer

Because the covered candle does not have a continuous supply of air. The oxygen inside the tumbler is soon used up, and without oxygen the flame is extinguished.

Wax + Oxygen (from air) → Carbon dioxide + Water vapour + Heat + Light
Under the tumbler: oxygen finishes → reaction stops → flame goes out
Why it happens: The component of air that supports combustion is oxygen. As the covered candle burns, it turns the carbon of the wax into carbon dioxide, which collects inside the tumbler. Carbon dioxide does not support burning, so as oxygen is replaced by carbon dioxide, the flame weakens and is put out.
Q2.
This can be confirmed by the presence of carbon dioxide in the glass tumbler inverted on the candle. How can you test the presence of carbon dioxide gas?
Answer

By the lime water test. Pour a small amount of freshly prepared lime water into the petri dish (or into the tumbler as soon as the candle goes out) and swirl it gently. The lime water turns milky, which confirms that carbon dioxide is present.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
   (Lime water, clear)               (white, insoluble → milky)
Why it happens: The carbon dioxide inside the tumbler was formed by the carbon of the wax joining with the oxygen of the air. Because calcium carbonate does not dissolve in water, the tiny white grains stay suspended and the lime water looks milky — a change we can see with our eyes, which is why it makes such a useful test.
Q3.
If a person’s clothes catch fire, what is the best way to extinguish the fire?
Answer

Wrap a blanket or a thick cloth tightly around the person. This cuts off the supply of air, and the fire is extinguished.

  • Make the person stop, drop and roll on the ground — rolling smothers the flame in the same way.
  • Water may also be poured over the person, if it is at hand.
  • Call for help and take the person to a doctor even if the burn looks small.
Why it happens: Fire needs three things — fuel, oxygen and heat. The clothes are the fuel and cannot be removed instantly, so the quickest thing we can take away is the oxygen. A blanket forms an airtight cover between the burning cloth and the air, and the flame dies for the same reason the covered candle did.
Caution: Never use a synthetic blanket or cloth to put out a fire. Synthetic fibres melt in the heat and stick to the skin, causing far worse injuries. Use cotton, wool or a jute sack. Also, never let the person run — running fans the flame with fresh air.
Q4.
We also know that paper is a combustible substance, but we can keep it in the air for any length of time without it catching fire. What else is needed to start combustion?
Answer

Heat — enough heat to raise the substance to its ignition temperature, the minimum temperature at which it catches fire.

So combustion needs three things together, shown as the fire triangle:

🔥 OXYGEN HEAT FUEL (combustible substance) Remove any one side and the fire stops
The fire triangle (Fig. 5.8): a fire needs fuel, oxygen and heat at the same time.
  1. A combustible substance, also called fuel — paper, wood, cotton, kerosene.
  2. Oxygen — supplied by the air.
  3. Heat that allows the fuel to reach its ignition temperature.
Why it happens: Paper lying on a table already has two of the three — it is the fuel, and air is all around it. What is missing is the third. Room temperature is far below the ignition temperature of paper, so it never catches fire on its own. A lighted matchstick, or sunrays focused with a lens, supplies that missing heat.
Tip: Every way of putting out a fire removes one side of this triangle — a blanket or sand removes oxygen, water removes heat, and closing the regulator of a gas cylinder removes the fuel.
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