Important Questions for Class 9 Chapter 2 - Is Matter Around Us Pure

Important questions based on NCERT syllabus for Chapter 2 - Is Matter Around Us Pure:

Question-1: What are the advantages of preparing solutions?

Solution: Advantages of preparing solutions:

i.To carry out reactions: For a reaction to take place, reactant molecules must come close together. In solutions, reactions take place faster because reactants are in ionic or molecular form and are close to each other.
ii. As dissolving medium: Certain medicines can only be administered to patients in solution form only. For example, saline solution is given to patients suffering from dehydration.

Question-2: Discuss the effect of pressure on solubility of gases in liquid? When we open the cap of a cola drink (or any carbonated beverage), why does gas bubbles come out?

Solution: Effect of pressure on solubility:
Solubility of gases increases with increase in pressure and solubility of gases decreases with decrease in pressure.

A cola (or carbonated) beverage is produced by dissolving carbon dioxide in the drink solution under pressure. An increase in pressure increases the solubility of gases. Therefore, more carbon dioxide dissolves at the higher pressure. When this pressure is suddenly released, e.g. by removing the cap of the bottle, the solubility of carbon dioxide decreases and it comes out in as bubbles from the solution.

Question-3: How is crystallization better than evaporation?

Solution: i. The process of evaporation give residues which may contain impurities. When there is a requirement to obtain solid without impurity, crystallization method is preferred.
ii. In general practice, impure solids are obtained by evaporation and are further purified by crystallization. e.g. salt obtained from sea water is further purified by this method.
iii. In some cases e.g. sugar crystal cannot be obtained by simple evaporation technique because sugar gets charred on heating to dryness. In such cases, crystallization technique is applied.