Chapter 02 Acids, Bases and Salts Questions-03
QUESTIONS
1. Why do $HCl, HNO_3$, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
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Answer
The dissociation of $HCl$ or $HNO 3$ to form hydrogen ions always occurs in the presence of water. Hydrogen ions $(H^{+})$ combine with $H 2 O$ to form hydronium ions $(H3O+$ ).
The reaction is as follows:
$ \begin{aligned} & HCl _{(a q)} \longrightarrow H^{+}+Cl^{-} \\ & H^{+}+H_2 O \longrightarrow H_3 O^{+} \end{aligned} $
Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.
2. Why does an aqueous solution of an acid conduct electricity?
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Answer
Acids dissociate in aqueous solutions to form ions. These ions are responsible for conduction of electricity.
3. Why does dry $HCl$ gas not change the colour of the dry litmus paper?
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Answer
Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain $H^{+}$ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither $HCl$ is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.
Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
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Answer
Exothermic reaction: It is a reaction in which energy is emitted in the form of light or heat. Energy is released to the environment as the reaction proceeds.
The reaction of acid and water is an exothermic reaction since a lot of heat is liberated during the reaction. This heat changes some water to steam explosively which can splash the acid on our face or clothes and cause acid burns. Therefore,
- While diluting an acid, it is preferred that the acid is added to water rather than the water being added to the acid.
- Adding water to a concentrated acid releases a large amount of heat, which can cause an explosion and acid burns on the skin, clothing, and other body parts.
Hence, adding acid to water is safe, but adding water to acid is not.
5. How is the concentration of hydronium ions $(H_3 O^{+})$affected when a solution of an acid is diluted?
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Answer
When an acid is diluted, the concentration of hydronium ions ( $H 3 O+)$ per unit volume decreases. This means that the strength of the acid decreases.
6. How is the concentration of hydroxide ions $(OH^{-})$affected when excess base is dissolved in a solution of sodium hydroxide?
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Answer
The concentration of hydroxide ions $(OH^{-})$would increase when excess base is dissolved in a solution of sodium hydroxide.
