Question

Rusting of iron is a

• A. physical change
• B. chemical change
• C. periodic change
• D. fast change

Answer 1

The correct option is B

chemical change

Correct option explanation:

(B) chemical change:

• During a chemical change, the substances undergo a chemical reaction.
• As a result of this, new substances (products) are formed which differ in their chemical composition from the original substances.
• For example, when an object made of iron$\left(\mathrm{Fe}\right)$ comes in contact with moist air for some time, it undergoes a chemical change and forms iron oxide, a reddish-brown powdery substance called rust($\left({\mathrm{Fe}}_2{\mathrm{O}}_3\right)$
• $\underset{\mathrm{iron}}{4\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{oxygen}\mathrm{from}\mathrm{moist}\mathrm{air}}{3{\mathrm{O}}_2\left(\mathrm{g}\right)}\stackrel{}{\to }\underset{\mathrm{rust}}{2{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)}$
• Iron oxide (rust) is a new substance and its properties are different from that of iron.
• Iron oxide (rust) cannot be changed back into iron, hence it is a permanent and irreversible change.

Incorrect option explanation:

(A) physical change:

• When a substance may undergo a change in some of its physical properties like colour, size, or shape, but does not undergo any change in its chemical composition is known as physical change.

(C) periodic change:

• Changes that take place again and again after a fixed interval of time are called periodic changes.

(D) fast change :

• These changes occur very fast in a short period of time.
• Some changes may occur even in a split of second.

Therefore, (B) option chemical change is the correct answer.