EN हिं

NCERT Solutions Curiosity Chapter 5 Is the Presence of Air Enough for Combustion? — Activity 5.6: Let us investigate

Book page 64 Updated on2026-09-05

Q1.
Hold a piece of paper with a pair of tongs and bring a lighted matchstick to it. It quickly catches fire. Do we say that we need a fire to start the burning process?
Answer

No. What we really need is heat, not a flame. A flame is only one convenient way of supplying that heat.

Why it happens: The flame of the matchstick is already far hotter than the ignition temperature of paper, so the paper is heated up to that temperature in a fraction of a second and catches fire at once. But the very next step of this activity proves that a flame is not necessary — sunrays focused by a magnifying glass, with no fire anywhere near, set the paper burning just as surely.
Caution: Perform this activity only under the supervision of your teacher or an adult, and always hold the paper with tongs.
Q2.
Take another piece of paper. Using a magnifying glass, focus the sunrays to make the smallest and brightest spot on the paper, as shown in the Fig. 5.7a. Hold it there for some time. What do you observe?
Answer

For a while nothing seems to happen. Then the bright spot turns brown, the paper starts to emit smoke, and finally it catches fire (Fig. 5.7b) — even though no flame was ever brought near it.

Sunrays spread over the lens → focused into one tiny bright spot
Spot heats the paper → temperature of the paper rises steadily
Temperature reaches the ignition temperature → smoke → fire
Why it happens: A magnifying glass collects the sunlight falling on its whole surface and brings it to a single small point. All that energy now heats a very small patch of paper, so the temperature of that patch climbs quickly. Thus a substance can burn even without fire.
Check it yourself: The nearer the spot is to a sharp, tiny point of light, the faster the paper burns. A big blurred spot spreads the same energy over a larger area and the paper only gets warm.
Q3.
We observe that the paper starts to emit smoke, and then catches fire (Fig. 5.7b). Thus, we find that a substance can burn even without fire. How do we explain this change?
Answer

By the idea of ignition temperature.

Focusing the sunrays heats the paper. Its temperature keeps rising with time. After some time the paper becomes so hot that it starts burning. The minimum temperature at which a substance catches fire is called its ignition temperature.

CaseSource of heatHow long it takesReason
Lighted matchstick brought to paperFlameAlmost at onceThe flame is already hotter than the ignition temperature of paper
Sunrays focused by a lensConcentrated sunlightAfter some timeThe paper has to be heated gradually up to its ignition temperature
Why it happens: Every combustible substance has its own ignition temperature. Below it, the substance simply gets warm; at or above it, the substance begins to burn. This is why a piece of paper can sit in air for years without catching fire — it never reaches its ignition temperature.
Was this helpful? Report an error