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
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A student tests an unknown solution with red litmus paper and observes that it turns blue. What can be concluded about the nature of the solution?
Q2 · Acids, Bases and Salts (Class 10) · MEDIUM
The pH of lemon juice is approximately 2.2. If the pH of a soap solution is 10, how many times more concentrated are the hydrogen ions in lemon juice compared to the soap solution?
Q3 · Acids, Bases and Salts (Class 10) · MEDIUM
When dilute hydrochloric acid is added to sodium carbonate, a gas is evolved. What happens when this gas is passed through freshly prepared lime water?
Q4 · Acids, Bases and Salts (Class 10) · MEDIUM
A salt X when dissolved in water gives a solution with pH less than 7. The salt X could be formed by the neutralization of which acid and base combination?
Q5 · Acids, Bases and Salts (Class 10) · HARD
Plaster of Paris has the chemical formula CaSO4.1/2H2O. When 29 kg of gypsum (CaSO4.2H2O) is heated to 373 K, what mass of Plaster of Paris will be obtained? (Given: Molar mass of CaSO4 = 136 g/mol)
**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]
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.