Rusting and Combustion
Class 7 Science · Chapter 5: Changes Around Us: Physical and Chemical · NCERT Curiosity section 5.3
Learn
An old iron gate near the school has a flaky, reddish-brown coating. Neel scrapes a little off: it is not iron any more but rust, a new substance called iron oxide. Iron reacts with oxygen and moisture in the air: rusting is a chemical change.
iron + oxygen (from air) + water → iron oxide (rust)
With a teacher, the friends hold a strip of magnesium ribbon in tongs over a flame. It burns with a dazzling white light and leaves a white powder, magnesium oxide. Burning a substance in oxygen, giving out heat and light, is combustion. Wood, paper, kerosene, LPG and cotton are combustible; stone and glass are not.
magnesium + oxygen → magnesium oxide + heat + light
Meher covers a burning candle with a glass tumbler: soon the flame dies, because the oxygen runs out. Combustion needs three things together, the fire triangle: a fuel, oxygen, and enough heat to reach the fuel's ignition temperature (the lowest temperature at which it catches fire). Even sunlight focused by a magnifying glass can heat paper to that point.
To put out a fire, remove one side of the triangle: water cools it, sand or a blanket cuts off air, and closing the gas knob removes the fuel. If someone's clothes catch fire, wrap them in a thick cotton blanket, never a synthetic one. Some living things, like fireflies, make light by a chemical change with almost no heat: bioluminescence.
A burning candle shows both kinds of change. Wax melting, rising up the wick and turning to vapour are physical; the vapour burning to give carbon dioxide, heat and light is chemical. A famous scientist once gave a whole series of lectures just on the science of a candle!
Explore: the life of a candle flame.
- Put the steps in order.
Remember
- Rusting forms iron oxide; burning magnesium forms magnesium oxide: both are chemical changes.
- Fire triangle: fuel, oxygen and heat (ignition temperature); remove one to stop a fire.
Quick check
Sand thrown on a small fire puts it out mainly because it …
- cuts off the supply of air (oxygen)
- is a fuel
- raises the temperature
- makes carbon dioxide
Answer
cuts off the supply of air (oxygen). Without oxygen, combustion cannot continue.
Think about it
Why does a matchstick light when rubbed on the box, but not when just left on it?
One answer
Rubbing produces enough heat to raise the match head to its ignition temperature. Lying still, it never gets that hot.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.