Question

Which of the following is a chemical change:
A.Melting of ice
B.Boiling of water
C.Rusting of iron
D.Tearing a paper

Answer 1

To answer this question, you should recall the concept of chemical change and physical change. A substance is said to undergo a physical change when only the physical properties of the reactant change while chemical change refers to a substance combines with another to form a new substance

Complete step by step answer:
Physical properties refer to the shape, size, colour, state or appearance of the substance. In a physical change chemical composition remains intact and is usually temporary. Chemical changes are irreversible except by further chemical reactions.
Rusting of iron is a chemical reaction which happens when iron comes in contact with air or water. The reaction behaves as an electrochemical cell reaction. In this reaction iron loses electrons and gets converted to ferrous state thus behaving as anode. The electrons lost from this atom of iron will move to the other side where they combine with protons released by exposed water.
The reactions taking place are:
$F{e^{2 + }}\; + {\text{ }}3{O_2}\; \to {\text{ }}2F{e_2}{O_3}$
$F{e_2}{O_3}\; + {\text{ }}x{H_2}O{\text{ }} \to {\text{ }}F{e_2}{O_3}.{\text{ }}x{H_2}O{\text{ }}\left( {{\text{rust}}} \right)$

Therefore, we can conclude the correct answer to this question is option C.

Note:
You should know about tarnishing along with rusting of iron. We know that tarnish is a type of chemical reaction formed as a product when a metal and a nonmetal compound, especially oxygen and sulfur dioxide react and undergo oxidation. The reaction for tarnishing can be represented as:
$8Ag + 4H{S^ - } \to {\text{ }}4A{g_2}S{\text{ }} + {\text{ }}2{H_2}\; + {\text{ }}4{e^ - }$
${O_2}\; + {\text{ }}2{H_2}O{\text{ }} + {\text{ }}4{e^ - }\; \to {\text{ }}4O{H^ - }$
Silver tarnish can be considered as a type of corrosion, but unlike rust, the underlying metal remains undamaged. The process of tarnishing is fastened through tap water contact as it has the presence of chlorine), various food items, perfume and hair spray etc. For slowing down the process of tarnishing, silver ornaments should be cleaned regularly