Corrosion

Rust on iron is the best-known case. Silver turns black in air, and copper slowly gains a green coat. Each is the metal reacting with something in the air.

Iron needs both air and water to rust. It can be protected by painting, oiling, greasing, galvanising with zinc, chrome plating, or making an alloy. Aluminium protects itself: a thin layer of oxide forms on it and stops further attack.

A small example

Put a clean iron nail in each of 3 test tubes. Tube A has plain water and air. Tube B has boiled water with oil on top, so no air gets in. Tube C has dry air, kept dry by a drying agent. After a few days, only the nail in tube A has rusted. It is the only one with both air and water.

Where marks go

CBSE's 2026-27 Science sample paper tests this twice. Question 27 asks why an iron bridge near the sea rusts faster than one in dry hills. The 2026-27 Science marking scheme gives 1 mark for moisture: sea air is humid, and moisture speeds up rusting.

Question 23 asks about aluminium wires. The scheme's answer is that they do not corrode easily because aluminium oxide forms a protective layer. So the marked reason names the cause, not just the result. See where the marks go in CBSE Science.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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