Combustion

What is combustion?

Recall burning a magnesium ribbon in the chapter on metals and non-metals. The ribbon burns with a dazzling light and leaves a white powder, magnesium oxide. A new substance forms, so it is a chemical change, and heat and light are given out as well. In short: magnesium + oxygen → magnesium oxide + heat + light. When a substance reacts with oxygen and gives out heat and/or light, the reaction is called combustion. Wood, paper, cotton and kerosene can burn like this, so they are combustible substances.

Burning needs oxygen

Activity 5.5 tests whether air is needed. Light two identical candles on petri dishes and cover one with a glass tumbler. The open candle keeps burning, but the covered one goes out after some time, because it gets no fresh supply of air. The part of the air that supports burning is oxygen. To confirm it, pour a little lime water into the dish under the tumbler: it turns milky, so carbon dioxide has formed. This carbon dioxide came from the carbon in the wax and the oxygen in the air, which shows that combustion uses oxygen.

Fire safety

If someone’s clothes catch fire, wrap a blanket or a thick cloth around the person at once. Without a fresh supply of air, the fire dies out. A synthetic blanket or cloth must never be used, because it can melt and stick to the skin.

Light without heat

On summer evenings you may see fireflies glowing in a garden or field. Their light comes from a chemical change too, but unlike a flame it gives no heat. Light made this way by living things is called bioluminescence.

Ignition temperature

Paper burns easily, yet a sheet can lie in the open air for years and never catch fire. What else is needed? In Activity 5.6, which needs a teacher or an adult nearby, a lighted matchstick is brought to a piece of paper held in tongs, and the paper soon catches fire. Now focus sunlight on another piece with a magnifying glass, making the smallest, brightest spot, and hold it there. The paper starts to smoke and then catches fire, without any flame. The focused sunlight heats the paper, its temperature keeps rising, and at last it is hot enough to burn. The lowest temperature at which a substance catches fire is its ignition temperature. The matchstick flame was already hotter than that, which is why the paper caught fire almost at once.

The fire triangle

So combustion needs three things, shown as the fire triangle: a combustible substance, also called a fuel; oxygen; and heat that lets the fuel reach its ignition temperature. Remove any one side and the fire cannot continue, which is why a blanket that cuts off the air puts out burning clothes.

What a burning candle shows

When a candle burns, the flame’s heat melts the wax, the liquid wax climbs the wick and turns into vapour, and melted wax that drips down solidifies again: all physical changes. The wax vapour then burns to produce the flame, a chemical change. So a burning candle shows physical and chemical changes together. In the nineteenth century the scientist Michael Faraday gave a series of lectures, The Chemical History of a Candle, using the candle to explain melting, vaporisation and combustion.

Why it matters

Understanding combustion helps us use fuels well and put out fires safely. How chemical changes differ from physical ones is explained in physical and chemical changes, and which changes can be undone in reversible and irreversible changes. Answers to every activity and question are in the NCERT solutions for Chapter 5; see all of the Chapter 5 notes.