Q. If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

• (a) Wash the hand with saline solution.
• (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate.
• (c) After washing with plenty of water apply solution of sodium hydroxide on the hand.
• (d) Neutralise the acid with a strong alkali.

Ans. (b) Wash the hand immediately with plenty of water and apply a paste of sodium hydrogencarbonate.

Explanation : If a few drops of a concentrated acid accidentally spills over the hand of a student then he should wash his hands immediately with plenty of water as it will dilute the acid and wash it out. Any remaining acid should be neutralised by applying the paste of sodium hydrogencarbonate.

Q. Which one of the following types of medicines is used for treating indigestion?

• (a) Antibiotic
• (b) Analgesic
• (c) Antacid
• (d) Antiseptic

Ans. (c) Antacid

Explanation : Antacids such as sodium bicarbonate and magnesium hydroxide etc are used in treating indigestion. They are basic in nature and reacts with excess acid present in the stomach and neutralises it.

Q. Which of the following is used for dissolution of gold?

• (a) Hydrochloric acid
• (b) Sulphuric acid
• (c) Nitric acid
• (d) Aqua regia

Ans. (d) Aqua regia

Explanation : Aqua regia is a mixture of hydrochloric acid and nitric acid in the ratio 3 : 1 and it can dissolve noble metals such as gold, palladium, and platinum, which however, not soluble in either of the acids alone.

Q. During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to:

• (a) Absorb the evolved gas
• (b) Moisten the gas
• (c) Absorb moisture from the gas
• (d) Absorb Cl ions from the evolved gas

Ans. (c) Absorb moisture from the gas

Explanation : Calcium chloride is used as an absorbent in the guard tube during the preparation of hydrogen chloride gas on a humid day as it absorbs the moisture present in the air and gives out white fumes on reacting with moisture.

Q. Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Ans. Acids like HCl, HNO3, etc; dissociate to form hydrogen ions. These hydrogen ions (H+) combine with H2O to form hydronium ion(H3O+). This H+ or H3O+ is responsible for acidity. It can be represented as:
HCl + H2O → H+ + Cl
H+ + H2O → H3O+

Aqueous solutions of glucose and alcohol contain hydrogen, but these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.

Q. Why does dry HCl gas not change the colour of the dry litmus paper?

Ans. Dry HCl gas does not contain H+ ions. Acids give hydrogen ions only in the aqueous solution. In case of dry litmus paper and dry HCl, water is not present. Therefore, dry HCl will not dissociate into hydrogen ions H+ and the colour of the litmus paper does not change.

Q. Why does an aqueous solution of an acid conduct electricity?

Ans. When electricity is passed through an aqueous solution of an acid, it dissociates in aqueous solution to form H+ and OH ions. The H+ ions reach the cathode and each H+ ion picks up one electron from the cathdoe to form H2 gas. Because of this reaction, an aqueous solution of acid conducts electricity.

Q. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans. The process of dissolving an acid or a base in water is a highly exothermic in nature. The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Hence, it is recommended that the acid should be added to water and not water to the acid.

Q. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans. When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.
Q. What effect does the concentration of H+(aq) ions have on the nature of the solution?
Ans. With increase in H+ ion concentration, the solution becomes more acidic, while a decrease in the concentration of H+ ion causes an increase in the basicity of the solution.

Q. Do basic solutions also have H+ (aq) ions? If yes, then why are these basic?

Ans. Yes, basic solution also has H+ (aq) ions. But the concentration of H+ ions is less as compared to the concentration of OH ions that makes the solution basic.

Q. If someone is suffering from the problem of acidity after overeating, which of the following would you suggest as remedy ? Lemon juice, Vinegar, Baking soda solution Give reason for your choice.

Ans. Baking soda solution. The solution being basic in nature, neutralises excess acid in the stomach.

Q. How is the neutralisation of a carbonate with an acid different from the neutralisation of an oxide or a hydroxide?

Ans. Neutralisation of a carbonate with an acid produces carbon dioxide gas but not with an oxide or hydroxide.

Q. Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it, as shown in figure below.

Ans. Neutralisation of a carbonate with an acid produces carbon dioxide gas but not with an oxide or hydroxide.

(i) What will be the action of gas on

• (a) dry litmus paper?
• (b) moist litmus paper?

(ii) Write a balanced chemical equation for the reaction taking place.

Ans. (i) (a) There will be no action on dry litmus paper.

(b) Since the gas is sulphur dioxide (SO2), it turns moist blue litmus paper to red because sulphur dioxide reacts with moisture to form sulphurous acid.

(ii) The balanced chemical reaction can be given as:

$$2H_2O(l) \xrightarrow{Electrolysis} 2H_2(g) + O_2(g)$$

These reactions require a source of energy to proceed. In case of combination reactions, two or more substances combine to give a new substance with the release of energy. Thus, both the reactions are opposite.

$$S(s) + O_2(g) \longrightarrow \underset{\text{Sulphur dioxide}}{SO_2(g)} \\ SO_2(g) + H_2O(s) \longrightarrow \underset{\text{Sulphurous Acid}}{H_2SO_3(aq)}$$