Effect of Air and Water on Metals

Class 7 Science · Chapter 4: The World of Metals and Non-metals · NCERT Curiosity section 4.2–4.3

Learn

Iron left outside grows brown flakes. Meher tests three clean nails for a week. A: with silica gel in a capped bottle (dry air only). B: under boiled, cooled water with oil on top (water, no air). C: half in water in an open bottle (air and water). Only C turns brown.

So iron needs both air and water (moist air) to form the brown deposit called rust; the process is rusting. Copper gets a green coat and silver a black one: such slow damage of metals by air, water and other substances is corrosion. Painting, oiling, greasing and a zinc coating (galvanisation) protect iron. Rust costs India huge sums every year.

iron + air + water → rust

The Iron Pillar of Delhi, made in the time of Chandragupta II more than 1600 years ago, is about 8 m tall, weighs over 6000 kg, and has hardly rusted: a sign of ancient India's skill in metal technology.

A teacher burns a clean magnesium ribbon: a dazzling white flame leaves a white powder, magnesium oxide. Dissolved in water it turns red litmus blue: metal oxides are generally basic. Sodium reacts so strongly with air and water that it is kept under kerosene.

magnesium + oxygen → magnesium oxide (basic)

Explore: the rusting experiment.

1. Put the steps in order.

Remember

• Iron rusts only with both air and water; rusting is a kind of corrosion.
• Metals burn to form basic oxides; sodium is stored in kerosene.

Quick check

Which iron nail rusts: in dry air, under oily boiled water, or half in water in an open bottle?

1. half in water in an open bottle
2. in dry air
3. under oily boiled water
4. all three

Think about it

Three iron nails are dipped in oil, water and vinegar. Which will not rust, and why?

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.