Question

Which one of the following processes involve chemical reactions?
A.Storing of oxygen gas under pressure in a gas cylinder
B.Liquefaction of air
C.Keeping petrol in a china dish in the open
D.Heating copper wire in presence of air at high temperature

Answer 1

Hint: When copper is heated in presence of air very slowly at high temperature, a layer of brown black copper oxide is formed which protects the copper from extensive corrosion. The chemical equation for this is: $2Cu(s) + {O_2}(g) \to 2CuO(s)$

Complete answer:
A chemical reaction is a change in which one or more substance(s) or reactant(s) react to form a new substance(s) with entirely different properties.
When copper is heated in the presence of air in a very high temperature, a chemical reaction takes place. In this situation, the highly heated copper (Cu) reacts with oxygen (O2​) of the air, copper undergoes an oxidation reaction to give out a new product, that is a thin layer of copper oxide (CuO) on the surface of metallic copper. This formation of a thin layer of copper oxide is certainly a chemical reaction.
The chemical equation for this is:
$2Cu(s) + {O_2}(g) \to 2CuO(s)$
In the first three options given here, there is no involvement of chemical reaction.
Hence, Heating copper wire in the presence of air at high temperature is a process that involves chemical reaction.

Therefore, the correct answer is option (D).

Note: The substance (or substances) that are initially involved in a chemical reaction are known as reactants or reagents. Chemical reactions are mainly characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants. But chemical reactions must be differentiated from the physical changes. Physical changes include changes of state, like ice melting to water and water evaporating to vapour etc. If a physical change occurs, then the physical properties of a substance changes, but its chemical identity remains the same.