Study Notes: Acids, Bases and Salts (Class 10)

Overview

Acids, bases and salts form the backbone of chemical reactions in everyday life and industrial processes. This topic appears consistently in SOF NSO with 3–5 questions testing pH scale understanding, indicator changes, neutralization reactions and properties of common salts. Students must master the chemical behavior of acids and bases, recognize acid-base indicators, calculate pH values and identify important salts like washing soda, baking soda and bleaching powder with their chemical formulas and uses.

The topic integrates practical chemistry with theory. Expect questions on real-world applications — using antacids for acidity, understanding why lemon juice removes rust, or why soap feels slippery. Strong performance requires memorizing key formulas, understanding neutralization stoichiometry and recognizing pH ranges of common substances. The Achievers Section often presents pH calculation challenges or asks students to predict the outcome of mixing specific acids with bases.

Key Concepts

• **Acids** release hydrogen ions (H⁺) in aqueous solution, taste sour, turn blue litmus red and react with metals to produce hydrogen gas. Examples: HCl, H₂SO₄, HNO₃, CH₃COOH.

• **Bases** release hydroxide ions (OH⁻) in solution, taste bitter, feel slippery, turn red litmus blue and neutralize acids. Water-soluble bases are called alkalis. Examples: NaOH, KOH, Ca(OH)₂, NH₄OH.

• **pH scale** measures acidity/basicity from 0–14. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Each unit change represents a tenfold change in H⁺ concentration.

• **Neutralization** occurs when an acid reacts with a base to form salt and water: Acid + Base → Salt + Water. Heat is released (exothermic reaction).

• **Indicators** are substances that show different colors in acidic and basic solutions. Litmus (red in acid, blue in base), phenolphthalein (colorless in acid, pink in base), methyl orange (red in acid, yellow in base).

• **Strength of acids/bases** depends on ionization degree. Strong acids/bases ionize completely (HCl, NaOH); weak acids/bases ionize partially (CH₃COOH, NH₄OH).

• **Salts** are ionic compounds formed during neutralization. Normal salts (complete neutralization), acidic salts (partial neutralization of polybasic acid), basic salts (partial neutralization of polyacidic base).

• **Water of crystallization** is the fixed number of water molecules present in one formula unit of a salt. Example: CuSO₄·5H₂O (blue vitriol), Na₂CO₃·10H₂O (washing soda).

Formulas / Key Facts

**pH Calculation:**

• pH = −log₁₀[H⁺]
• [H⁺] = 10⁻ᵖᴴ
• pH + pOH = 14 at 25°C

**Important Acids:**

• Hydrochloric acid — HCl (stomach acid, cleaning agent)
• Sulfuric acid — H₂SO₄ (battery acid, king of chemicals)
• Nitric acid — HNO₃ (fertilizers, explosives)
• Acetic acid — CH₃COOH (vinegar, 5–8% solution)
• Carbonic acid — H₂CO₃ (soft drinks)

**Important Bases:**

• Sodium hydroxide — NaOH (caustic soda, soap making)
• Potassium hydroxide — KOH (caustic potash)
• Calcium hydroxide — Ca(OH)₂ (slaked lime, whitewash)
• Magnesium hydroxide — Mg(OH)₂ (milk of magnesia, antacid)
• Ammonium hydroxide — NH₄OH (aqueous ammonia, window cleaner)

**Important Salts:**

• Sodium chloride — NaCl (common salt, preservative)
• Sodium carbonate — Na₂CO₃·10H₂O (washing soda, water softener)
• Sodium hydrogen carbonate — NaHCO₃ (baking soda, antacid, fire extinguisher)
• Calcium carbonate — CaCO₃ (limestone, marble, chalk)
• Calcium sulfate — CaSO₄·½H₂O (plaster of Paris)
• Calcium oxychloride — CaOCl₂ (bleaching powder, disinfectant)
• Copper sulfate — CuSO₄·5H₂O (blue vitriol, fungicide)

Worked Examples

**Example 1: pH Identification** *Question:* A solution has [H⁺] = 10⁻⁹ M. What is its pH and nature?

*Solution:* pH = −log₁₀[H⁺] = −log₁₀(10⁻⁹) = 9

Since pH = 9 > 7, the solution is **basic**.

**Example 2: Neutralization Reaction** *Question:* Write the balanced equation when sulfuric acid reacts with sodium hydroxide. Identify the salt formed.

*Solution:* H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

The salt formed is **sodium sulfate (Na₂SO₄)**, a normal salt from complete neutralization of a dibasic acid with a monoacidic base.

**Example 3: Indicator Color Change** *Question:* A solution turns phenolphthalein pink and methyl orange yellow. What can you conclude?

*Solution:*

• Phenolphthalein pink → pH > 8.3 (basic)
• Methyl orange yellow → pH > 4.4 (neutral to basic)

Both indicators confirm the solution is **basic** (pH > 8).

**Example 4: Water of Crystallization** *Question:* Calculate the percentage of water in washing soda (Na₂CO₃·10H₂O). [Na = 23, C = 12, O = 16, H = 1]

*Solution:* Molecular mass of Na₂CO₃·10H₂O = 2(23) + 12 + 3(16) + 10[2(1) + 16] = 46 + 12 + 48 + 180 = 286

Mass of 10H₂O = 180

Percentage of water = (180/286) × 100 = **62.94%**

Common Mistakes

**Mistake 1: Confusing strong/weak with concentrated/dilute** Wrong thinking → "Concentrated acid is always a strong acid." Correct fix → Strength refers to ionization degree; concentration refers to amount of solute. Acetic acid can be concentrated but remains a weak acid because it ionizes partially.

**Mistake 2: Incorrect pH scale interpretation** Wrong thinking → "pH 6 is twice as acidic as pH 3." Correct fix → pH scale is logarithmic. pH 3 has 1000 times more H⁺ ions than pH 6 (each unit = 10× change).

**Mistake 3: Mixing up indicator color changes** Wrong thinking → "Litmus turns pink in base." Correct fix → Phenolphthalein turns pink in base; litmus turns blue in base. Red litmus → blue (base), blue litmus → red (acid).

**Mistake 4: Forgetting water of crystallization** Wrong thinking → "Formula of washing soda is Na₂CO₃." Correct fix → Always include water of crystallization: Na₂CO₃·10H₂O. Similarly, CuSO₄·5H₂O for blue vitriol, not just CuSO₄.

**Mistake 5: Incomplete neutralization equations** Wrong thinking → "HCl + NaOH → NaCl" (forgetting water) Correct fix → Always write: Acid + Base → Salt + Water. Complete equation: HCl + NaOH → NaCl + H₂O

Quick Reference

• **pH < 7:** Acidic; **pH = 7:** Neutral; **pH > 7:** Basic
• **Universal indicator color:** Red (0–3) → Orange (4–6) → Green (7) → Blue (8–11) → Violet (12–14)
• **Litmus:** Red in acid, blue in base
• **Phenolphthalein:** Colorless in acid, pink in base (pH > 8.3)
• **Common salt (NaCl):** Neutralization of HCl + NaOH
• **Baking soda (NaHCO₃):** Mild base, releases CO₂ with acid, used in fire extinguishers and antacids
• **Washing soda (Na₂CO₃·10H₂O):** Water softener, removes permanent hardness, pH ≈ 11
• **Bleaching powder (CaOCl₂):** Made from Cl₂ + dry slaked lime, disinfectant and bleaching agent
• **Plaster of Paris (CaSO₄·½H₂O):** Sets hard on adding water, used for casts and moulds