Question

If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.

(a) Do you think rust is different from iron?

(b) Can you change rust back into iron by some simple method?

(c) Do you think the formation of rust on iron is a chemical change?

(d) Give two other examples of a similar type of change.

Answer 1

(a) Yes, rust is iron oxide ($F{e}_2{O}_3$). Thus, rust and iron are not the same substance.

(b) No, rusting of iron is a chemical change because, in this reaction, a new substance, rust (iron oxide) is formed. It cannot be reversed by any method.

(c) Yes, rusting of iron is a chemical change. During the rusting of iron, it combines with the oxygen in the presence of water (moisture) to form a new compound ‘iron oxide’. This iron oxide is rust.

Iron + Oxygen + Water Iron oxide

(From damp air) (Rust)

It is a permanent change that cannot be reversed back. So, rusting of iron is a chemical change.

(d) Two other examples are:

• Curdling of milk

• Burning of magnesium ribbon to form magnesium oxide.