1. What are Acids, Bases and Salts?

Acids

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. They usually taste sour and change blue litmus to red.

Examples: Hydrochloric acid — HCl, Sulfuric acid — H₂SO₄, Nitric acid — HNO₃.

Bases

Bases are substances that produce hydroxide ions (OH⁻) in water. They usually feel soapy/slippery and change red litmus to blue.

Examples: Sodium hydroxide — NaOH, Calcium hydroxide — Ca(OH)₂.

Salts

A salt is formed when an acid reacts with a base (neutralisation), or when the hydrogen of an acid is replaced by a metal. Salts are ionic compounds made of a positive ion (cation) and a negative ion (anion).

Examples: Sodium chloride — NaCl, Copper sulfate — CuSO₄, Calcium carbonate — CaCO₃.

2. Physical properties (quick points)

• Acids: Sour taste, conduct electricity in solution, corrosive (concentrated acids).
• Bases: Bitter taste, slippery feel, conduct electricity in solution.
• Salts: Usually crystalline solids, often soluble in water, conduct electricity when molten or in solution.

Important safety note

Never taste chemicals in the lab. Always use safety gear (gloves, goggles) when handling concentrated acids or bases.

3. Chemical properties and reactions

Reaction with metals

Acids react with many metals to give a salt and hydrogen gas:

Metal + Acid → Salt + Hydrogen gas
Example: Zn + 2HCl → ZnCl₂ + H₂

Reaction with metal oxides

Acids react with metal oxides to form salt and water (neutralisation):

Metal oxide + Acid → Salt + Water
Example: CuO + 2HCl → CuCl₂ + H₂O

Reaction with carbonates and bicarbonates

Carbonates react with acids to produce a salt, carbon dioxide and water:

Carbonate + Acid → Salt + CO₂ + H₂O
Example: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

Neutralisation (acid + base)

Neutralisation reaction forms a salt and water:

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

4. pH Scale and Indicators

The pH scale ranges from 0 to 14 and measures how acidic or basic a solution is:

• pH < 7 : Acidic (strong acids can be pH 0–3)
• pH = 7 : Neutral (pure water)
• pH > 7 : Basic/alkaline

Common indicators:

• Litmus: red in acid, blue in base.
• Phenolphthalein: colourless in acid/neutral, pink in basic solution (pH > ~8.2).
• Methyl orange: red in acid, yellow in base (useful in titrations for strong acid–weak base).

Universal Indicator

Universal indicator gives a gradual colour change across the pH scale and helps estimate pH value approximately.

5. Preparation and Uses of Important Salts (NCERT focus)

Methods of Salt Preparation (short notes)

• By neutralisation (acid + base) — e.g., HCl + NaOH → NaCl + H₂O.
• By reaction of acid with metal — e.g., Cu + 2HCl → CuCl₂ + H₂.
• By reaction of acid with metal carbonate — e.g., CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.
• By precipitation (mixing two solutions where an insoluble salt forms).

6. Conceptual clarity — Frequently tested points

• Difference between strong and weak acids/bases (strength = degree of ionisation in water). Example: HCl is a strong acid, CH₃COOH (acetic acid) is a weak acid.
• pH and [H⁺] relationship: pH = −log₁₀[H⁺]. If [H⁺] = 10⁻³ mol L⁻¹, pH = 3.
• Salt formation and neutralisation — understand balanced equations and identify products.
• Indicator behaviour — know which indicator to use in titration (phenolphthalein vs methyl orange).

7. Quick Revision Checklist (pointwise)

1. Definitions: acid, base, salt — give one example each.
2. Properties: physical & chemical differences between acids and bases.
3. pH scale: know neutral, acidic and basic ranges and pH formula.
4. Indicatiors: litmus, phenolphthalein, methyl orange — colour changes.
5. Neutralisation reaction and common salt formation reactions.
6. Methods of preparation of salts — neutralisation, metal/acid, carbonate/acid.
7. Safety and daily-life examples (antacids, toothpastes, fertilizers).

8. Practice — Short questions (with suggested answers)

• Q: Give one chemical test to distinguish between an acid and a base. A: Use litmus paper — acid turns blue litmus red; base turns red litmus blue.
• Q: What is the pH of a neutral solution? A: pH = 7.
• Q: Write the reaction when hydrochloric acid reacts with sodium carbonate. A: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂.