Class 9 Chemistry Notes – Chapter 7

Subject: Chemistry

Class: 9th

Board: Punjab Board

Chapter 7: Acid Base Chemistry

Exercise: Multiple Choice Questions (MCQs)

(i) Which of the following is a strong acid?

Answer: (c) HCl

(ii) According to Arrhenius concept, an acid is a substance that produces:

Answer: (a) H⁺ ions in aqueous solution.

(iii) The conjugate acid of NH₃ is:

Answer: (b) NH₄⁺

(iv) Which of the following is a Lewis base?

Answer: (d) NH₃

(v) The pH of a neutral solution at 25°C is:

Answer: (b) 7

(vi) Which salt is acidic in aqueous solution?

Answer: (a) NH₄Cl

(vii) The process of neutralization between a strong acid and a strong base produces:

Answer: (c) Salt and water.

(viii) Which of the following indicators turns pink in a basic medium?

Answer: (b) Phenolphthalein

(ix) What is the pOH of a solution with [OH⁻] = 1 × 10⁻³ M?

Answer: (c) 3

(x) Rainwater is slightly acidic due to the dissolution of:

Answer: (b) Carbon dioxide.

Exercise: Questions for Short Answers

(i) Define Arrhenius acid and base with examples.

Answer: According to Arrhenius, an acid is a substance that dissociates in water to produce hydrogen ions (H⁺), such as HCl. A base is a substance that dissociates in water to produce hydroxide ions (OH⁻), such as NaOH.

(ii) What is the difference between Brønsted–Lowry acid and Lewis acid?

Answer: A Brønsted–Lowry acid is a proton (H⁺) donor, whereas a Lewis acid is an electron pair acceptor. Lewis acids include many species that do not contain hydrogen atoms.

(iii) Identify the conjugate acid-base pairs in the following reaction:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

Answer:

• Conjugate pair 1: NH₃ (base) and NH₄⁺ (conjugate acid).

• Conjugate pair 2: H₂O (acid) and OH⁻ (conjugate base).

(iv) Why is BF₃ considered a Lewis acid?

Answer: BF₃ is considered a Lewis acid because boron has an incomplete octet of 6 electrons and can accept a pair of electrons from a donor molecule.

(v) Calculate the pH of a 0.001 M HCl solution.

Answer:

Given:

[H⁺] = 0.001 M = 1 × 10⁻³ M

Formula:

pH = −log[H⁺]

Calculation:

pH = −log(1 × 10⁻³)

pH = 3

Exercise: Constructed Response Questions

(i) Can a substance act as both Brønsted–Lowry acid and base? Explain with an example.

Answer: Yes, amphoteric substances like water (H₂O) can act as both. Water can donate a proton to act as an acid or accept a proton to form a hydronium ion (H₃O⁺) to act as a base.

(ii) Why is pure water a weak electrolyte?

Answer: Pure water undergoes self-ionization to a very small extent, producing extremely low concentrations of H₃O⁺ and OH⁻ ions (1 × 10⁻⁷ M each at 25°C), resulting in low electrical conductivity.

(iii) Explain why aqueous solutions of salts can be acidic, basic, or neutral.

Answer: Salt solutions depend on the strength of their parent acids and bases. Salts of strong acids and strong bases yield neutral solutions (pH = 7). Salts of strong acids and weak bases yield acidic solutions (pH < 7), while salts of weak acids and strong bases yield basic solutions (pH > 7).

(iv) How do acid-base indicators work?

Answer: Acid-base indicators are weak organic acids or bases whose ionized and unionized forms possess distinctly different colours. Changing the pH of the solution alters the equilibrium between these forms, causing a visible colour change.

(v) What is the role of buffers in biological systems?

Answer: Buffer systems resist drastic changes in pH when small amounts of acids or bases are added, maintaining stable internal conditions critical for enzyme activity and cellular processes in living organisms, such as blood bicarbonate buffers.

Exercise: Descriptive Questions

(i) Compare Arrhenius, Brønsted–Lowry, and Lewis concepts of acids and bases.

Answer:

• Arrhenius Concept: Acids produce H⁺ ions and bases produce OH⁻ ions in aqueous solutions.

• Brønsted–Lowry Concept: Acids are proton (H⁺) donors and bases are proton acceptors. This concept applies in any medium.

• Lewis Concept: Acids are electron pair acceptors and bases are electron pair donors, providing the broadest definition.

(ii) Define the pH scale. How is it used to determine the acidity and alkalinity of solutions?

Answer: The pH scale is a logarithmic scale ranging from 0 to 14 used to measure the hydrogen ion concentration of a solution. A pH below 7 indicates acidity, with acidity increasing at lower values. A pH of 7 is neutral, and a pH above 7 indicates alkalinity, with alkalinity increasing at higher values.

(iii) Describe the preparation methods of salts in detail.

Answer: Salts can be prepared by several methods:

• Direct combination of elements: For example, burning a metal in a non-metal gas.

• Reaction of an acid with a metal: Produces salt and hydrogen gas.

• Neutralization reaction: An acid reacts with a base to produce salt and water.

• Reaction of an acid with metal oxides, hydroxides, or carbonates: Produces salts through suitable chemical reactions.

(iv) Explain neutralization reactions with examples.

Answer: A neutralization reaction occurs when an acid reacts with a base to produce salt and water. It involves the combination of H⁺ and OH⁻ ions to form H₂O.

Example:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

(v) Discuss the properties and uses of important mineral acids (HCl, H₂SO₄, HNO₃).

Answer:

• Hydrochloric Acid (HCl): Used in cleaning metals, manufacturing chlorides, and gastric digestion.

• Sulphuric Acid (H₂SO₄): Known as the king of chemicals, it is used in fertilizer manufacturing, lead-acid storage batteries, and petroleum refining.

• Nitric Acid (HNO₃): Used in producing fertilizers, explosives (such as TNT), and etching metals.

Exercise: Investigative Questions

(i) Antacids are commonly used to treat acid indigestion or heartburn. Explain the chemical reaction involved.

Answer: Acid indigestion is caused by excess hydrochloric acid in the stomach. Antacids contain mild bases such as magnesium hydroxide or aluminium hydroxide that react with excess HCl in a neutralization reaction to form salts and water, relieving acidity and raising stomach pH.

(ii) Acid rain is a major environmental concern. How does it form, and what are its harmful effects?

Answer: Acid rain forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), emitted from burning fossil fuels, dissolve in atmospheric moisture to form sulfuric acid and nitric acid.

Its harmful effects include:

• Lowering the pH of lakes and soils.

• Damaging forests and other vegetation.

• Corroding buildings and metallic structures.

• Leaching toxic heavy metals into water supplies.

Notes by Sir Dilawar

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