Acids, Bases and Salts
Overview
Acids, Bases and Salts forms a core chapter in the Chemistry portion of OTET Paper II Science. This topic bridges everyday observations (sour taste of lemon, slippery feel of soap) with fundamental chemical concepts. Questions typically test your understanding of definitions, properties, indicators, pH scale, and reactions involving acids, bases and salts.
For OTET, expect 2–4 questions from this topic. The examiner often focuses on identification using indicators, pH values of common substances, neutralisation reactions, and practical applications. Mastery here also supports related topics like metals-nonmetals and environmental chemistry.
Students must be able to distinguish between acids and bases using multiple criteria, interpret the pH scale correctly, write balanced equations for neutralisation, and connect classroom chemistry to real-life contexts like antacids, fertilisers, and food preservation.
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Key Concepts
- **Arrhenius Definition**: Acids release H⁺ ions in water; bases release OH⁻ ions in water. This is the working definition for school-level chemistry.
- **Indicators**: Substances that show different colours in acidic and basic solutions. Litmus, phenolphthalein, and methyl orange are the three you must know cold.
- **pH Scale**: A measure of hydrogen ion concentration on a scale of 0–14. pH 7 is neutral; below 7 is acidic; above 7 is basic/alkaline.
- **Neutralisation**: Acid + Base → Salt + Water. This exothermic reaction is the basis of antacid action and soil treatment.
- **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (CH₃COOH, H₂CO₃) ionise partially. Same logic applies to bases.
- **Salts**: Ionic compounds formed by neutralisation. Classified as normal, acidic, or basic salts depending on the parent acid and base.
- **Water of Crystallisation**: Some salts exist as hydrated crystals (e.g., CuSO₄·5H₂O — blue vitriol). Heating removes this water.
- **Dilution**: Always add acid to water, never water to acid. Dilution is exothermic and adding water to concentrated acid causes dangerous splattering.
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Formulas / Key Facts
| Concept | Key Fact | |---------|----------| | pH formula | pH = −log₁₀[H⁺]; for OTET, just know the scale interpretation | | Neutral pH | Pure water has pH = 7 at 25°C | | Acidic range | pH 0–6.9 (lower = more acidic) | | Basic range | pH 7.1–14 (higher = more basic) | | Litmus colours | Red litmus → Blue in base; Blue litmus → Red in acid | | Phenolphthalein | Colourless in acid; Pink in base | | Methyl orange | Red in acid; Yellow in base | | Neutralisation equation | HCl + NaOH → NaCl + H₂O | | Common strong acids | HCl (hydrochloric), H₂SO₄ (sulphuric), HNO₃ (nitric) | | Common weak acids | CH₃COOH (acetic/vinegar), H₂CO₃ (carbonic) | | Common strong bases | NaOH (caustic soda), KOH (caustic potash) | | Common weak bases | NH₄OH (ammonium hydroxide), Mg(OH)₂ (milk of magnesia) | | Baking soda | NaHCO₃ — sodium hydrogen carbonate | | Washing soda | Na₂CO₃·10H₂O — sodium carbonate decahydrate | | Plaster of Paris | CaSO₄·½H₂O — calcium sulphate hemihydrate |