Complete Chemistry notes on  Acids, Bases, and Salts for BPSC and Other Competitive Exams in 2025

Complete Chemistry notes on  Acids, Bases, and Salts for BPSC and Other Competitive Exams in 2025

General Science- 9 (Chemistry)

Acids, Bases, and Salts

Acids, bases, and salts are fundamental chemical compounds that are vital to both natural processes and industrial applications. Understanding their properties, reactions, and uses is essential for science-based competitive exams.

1. Acids

Acids are substances that typically donate protons (H⁺) or accept electrons.

Definitions:

• Arrhenius Definition: Acids produce H⁺ ions (or H₃O⁺ ions) in aqueous solutions.
  Example:
  HCl(aq) → H⁺(aq) + Cl⁻(aq)
• Brønsted-Lowry Definition: Acids are proton donors.
  Example:
  HCl + H₂O → H₃O⁺ + Cl⁻
• Lewis Definition: Acids are electron pair acceptors.
  (Note: Less emphasized in basic-level exams.)

General Properties of Acids:

1. Taste: Sour (e.g., lemon juice, vinegar). (Do not taste chemicals in labs.)
2. Corrosive: Strong acids can cause burns and damage materials.
3. Litmus Test: Turn blue litmus red.
4. Reaction with Metals: Produce hydrogen gas and salt.
   Acid + Metal → Salt + H₂
   Example:
   2HCl + Zn → ZnCl₂ + H₂
5. Reaction with Carbonates/Bicarbonates: Produce CO₂, water, and salt.
   Example:
   2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
6. Reaction with Bases: Form salt and water (neutralization).
   Acid + Base → Salt + Water

Strength of Acids:

• Strong Acids: Completely ionize in water.
  Examples: HCl, H₂SO₄, HNO₃
• Weak Acids: Partially ionize in water.
  Examples: CH₃COOH (acetic), H₂CO₃ (carbonic), citric acid

Common Acids & Their Uses:

• HCl: Found in stomach; used in cleaners, steel pickling
• H₂SO₄: “King of Chemicals”, used in batteries, fertilizers
• HNO₃: Used in fertilizers, explosives
• CH₃COOH: Vinegar; used in food
• Citric Acid: In citrus fruits; food preservative
• H₂CO₃: Present in carbonated drinks

2. Bases

Bases are substances that typically accept protons (H⁺) or donate electron pairs.

Definitions:

• Arrhenius Definition: Bases produce OH⁻ ions in aqueous solutions.
  Example:
  NaOH → Na⁺ + OH⁻
• Brønsted-Lowry Definition: Bases are proton acceptors.
  Example:
  NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
• Lewis Definition: Bases are electron pair donors.

General Properties of Bases:

1. Taste: Bitter (Never taste in lab)
2. Touch: Slippery or soapy
3. Corrosive: Strong bases can cause burns
4. Litmus Test: Turn red litmus blue
5. Reaction with Acids: Produce salt and water
6. Reaction with Non-metal Oxides:
   Example:
   2NaOH + CO₂ → Na₂CO₃ + H₂O

Strength of Bases:

• Strong Bases: Fully ionize in water
  Examples: NaOH, KOH, Ca(OH)₂
• Weak Bases: Partially ionize
  Examples: NH₄OH, Mg(OH)₂

Common Bases & Their Uses:

• NaOH: Caustic soda; used in soaps, paper, cleaners
• KOH: Caustic potash; used in soaps, batteries
• Ca(OH)₂: Slaked lime; used in whitewash, soil treatment
• Mg(OH)₂: Milk of Magnesia; antacid and laxative
• NH₄OH: Used in cleaners and fertilizers

3. Salts

Salts are ionic compounds formed by the neutralization reaction between acids and bases.

Formation:

Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H₂O

General Properties of Salts:

1. Ionic Compounds: Contain cations and anions
2. Solid State: Crystalline at room temperature
3. High Melting/Boiling Points
4. Solubility: Many are water-soluble
5. Conductivity: Molten or aqueous salts conduct electricity

Types of Salts:

• Neutral Salts: Strong acid + strong base (pH ≈ 7)
  Example: NaCl
• Acidic Salts: Strong acid + weak base (pH < 7)
  Example: NH₄Cl
• Basic Salts: Weak acid + strong base (pH > 7)
  Example: Na₂CO₃

Common Salts & Their Uses:

• NaCl: Table salt; used in food, chemical industry
• NaHCO₃: Baking soda; used in baking, antacids, fire extinguishers
• Na₂CO₃: Washing soda; used in detergents, glass, paper
• KNO₃: Used in fertilizers, fireworks, gunpowder
• CaSO₄·2H₂O: Gypsum; used in plaster, cement

4. pH Scale

The pH scale measures the acidity or basicity of a solution.

Key Features:

• Range: 0 to 14
• pH < 7: Acidic
• pH = 7: Neutral
• pH > 7: Basic (alkaline)
• Logarithmic Nature: Each unit change is a 10-fold change in H⁺ concentration
  (e.g., pH 3 is 10× more acidic than pH 4)

Importance of pH in Daily Life:

• Stomach Acid: pH 1.5–3.5 for digestion
• Blood pH: Maintained at 7.35–7.45; vital for survival
• Soil pH: Affects plant growth; lime is added to acidic soils
• Rainwater: Normally ~5.6 (due to CO₂); acid rain < 5.6 (due to SO₂, NO₂)
• Swimming Pools: pH control ensures comfort and sanitation

Indicators:

Used to detect pH changes:

• Litmus
• Phenolphthalein
• Methyl orange

Practice Questions (Objective)

1. Which of the following is a general property of acids?
   • A) Turn red litmus blue
   • B) Bitter taste
   • C) React with metals to produce hydrogen gas
   • D) Soapy feel

Rationale: Acids react with active metals like zinc or magnesium to release hydrogen gas

2. Which is a strong acid?
   • A) Acetic acid
   • B) Carbonic acid
   • C) Citric acid
   • D) Sulfuric acid

Rationale: Sulfuric acid (H2​SO4​), hydrochloric acid (HCl), and nitric acid (HNO3​) are common strong acids.

3. Acid + Metal Carbonate produces which gas?
   • A) Hydrogen
   • B) Oxygen
   • C) Carbon dioxide
   • D) Nitrogen

Rationale: The reaction of an acid with a metal carbonate or bicarbonate always produces carbon dioxide gas.

4. A substance that produces OH⁻ ions in water is called a:

• A) Acid
• B) Salt
• C) Base
• D) Indicator

Rationale: This is the Arrhenius definition of a base.

5. Which is a characteristic property of bases?

• A) Sour taste
• B) Turn blue litmus red
• C) Slippery feel
• D) Produce CO₂ with carbonates

Rationale: The soapy feel is a distinct characteristic of bases.

6. Milk of Magnesia contains:

• A) NaOH
• B) KOH
• C) Mg(OH)₂
• D) Ca(OH)₂

Rationale: Magnesium hydroxide (Mg(OH)2​) is the active ingredient in Milk of Magnesia, used to neutralize excess stomach acid.

7. In a neutralization reaction, the product formed besides water is:

• A) Oxide
• B) Element
• C) Salt
• D) Gas

Rationale: The fundamental product of an acid-base neutralization reaction is a salt and water.

8. Sodium chloride is formed from:

• A) H₂SO₄ + Ca(OH)₂
• B) HNO₃ + KOH
• C) HCl + NaOH
• D) CH₃COOH + NH₄OH

Rationale: Sodium chloride is formed from hydrochloric acid (HCl) and sodium hydroxide (NaOH).

9. A solution with pH 2 is:

• A) Neutral
• B) Weak base
• C) Strong acid
• D) Strong base

Rationale: pH values below 7 indicate acidity, with lower numbers indicating stronger acidity.

10. Normal pH of human blood is:

• A) 6.0 – 6.5
• B) 5.5 – 6.0
• C) 7.35 – 7.45
• D) 8.0 – 8.5

Rationale: Human blood pH is strictly regulated within a slightly alkaline range of 7.35 to 7.45.