Question

Give one example of each:
(a) Chemical reaction showing evolution of gas
(b) Change in substance’s colour during a chemical reaction.

Answer 1

Hint: We know that a chemical reaction is a chemical change in which the chemical composition of a substance is changed and one or more new substances with different chemical composition is formed. There are many observable changes take place during the chemical reaction such as evolution of gas, temperature change, change of colour etc.

Complete step by step answer:
(a)Let's understand the evolution of gas in a chemical reaction. Some chemical reactions produce gas such as reaction of zinc and sulphuric acid produces zinc sulphate and hydrogen gas. The formation of gas is indicated by the sign of $ \uparrow $.
The chemical equation for the above reaction is,
${\rm{Zn}} + {{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to {\rm{ZnS}}{{\rm{O}}_{\rm{4}}} + {{\rm{H}}_{\rm{2}}} \uparrow $
Here, the evolution of hydrogen gas takes place.
(b)Let’s understand the change of colour due to chemical reactions. Certain chemical reactions are characterized by a change in the colour of reactants. For example, when we add some pieces of iron to the blue colored copper sulphate solution, the colour of the solution turns light green.
${\rm{Fe}} + {\rm{CuS}}{{\rm{O}}_{\rm{4}}}\left( {{\rm{blue}}\,{\rm{colour}}} \right) \to {\rm{FeS}}{{\rm{O}}_{\rm{4}}}\left( {{\rm{green}}\,{\rm{colour}}} \right) + {\rm{Cu}}$

Note: Some other observable characteristics of chemical reaction are,
1) Formation of precipitate: Some chemical reaction forms insoluble solids (precipitates). Example, yellow colored precipitate of lead iodide is formed due to reaction of potassium iodide and lead nitrate.
2) Change of state: In many chemical reactions the formed product possesses a different state than the reactant. For example, burning of wax (solid) produces carbon dioxide (gas) and water (liquid).
3) Change of temperature: Some chemical reactions are characterized by change of temperature such as addition of water to quicklime produces slaked lime and a lot of heat.