Acids, Bases and Salts — RRB Group D Study Notes
Overview
Acids, Bases and Salts is a high-yield topic in the General Science (Chemistry) section of Railway Group D exams. Questions typically test basic definitions, pH scale interpretation, neutralisation reactions, properties of acids/bases, and identification of common household salts and their uses. This topic overlaps with everyday chemistry—think lemon juice, soap, antacids, baking soda—so examples are easy to relate to and remember.
Expect 2–3 direct questions on pH values, indicators, chemical formulas of common acids/bases/salts, or reactions. Mastery involves knowing the pH scale range, recognising strong vs weak acids/bases, writing neutralisation equations, and recalling real-world applications of salts like common salt (NaCl), washing soda (Na₂CO₃), and baking soda (NaHCO₃). Clarity on indicators (litmus, phenolphthalein, methyl orange) and their colour changes is also crucial.
Since this is conceptual rather than calculation-heavy, focus on definitions, properties, examples and classification. The exam rewards accurate recall and the ability to match substances to their acidic/basic/neutral nature.
Key Concepts
- **Acids** are substances that taste sour, turn blue litmus red, and release hydrogen ions (H⁺) in aqueous solution. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), acetic acid (CH₃COOH). Strong acids ionise completely; weak acids ionise partially.
- **Bases** taste bitter, feel slippery, turn red litmus blue, and release hydroxide ions (OH⁻) in water. Water-soluble bases are called alkalis. Examples: sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂). Strong bases dissociate fully; weak bases partially.
- **Salts** form when an acid reacts with a base in a neutralisation reaction. They are ionic compounds; common salt (NaCl) is the archetypal example. Salts can be neutral, acidic or basic depending on the parent acid and base.
- **pH scale** measures acidity or basicity on a 0–14 range. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic/alkaline. Each unit is a tenfold change in hydrogen ion concentration. Lower pH means higher acidity.
- **Indicators** change colour in acidic or basic media. Litmus (red in acid, blue in base), phenolphthalein (colourless in acid, pink in base), and methyl orange (red in acid, yellow in base) are standard lab indicators.
- **Neutralisation** is the reaction between an acid and a base to produce salt and water: Acid + Base → Salt + Water. The reaction is exothermic (releases heat). Example: HCl + NaOH → NaCl + H₂O.
- **Strength of acids/bases** depends on degree of ionisation, not concentration. Concentrated vinegar is still a weak acid; dilute HCl is still a strong acid.
- **Water of crystallisation** is water molecules trapped in the crystal structure of a salt. Example: hydrated copper sulphate (CuSO₄·5H₂O) is blue; anhydrous CuSO₄ is white.
Formulas / Key Facts
1. **Common acids:**
- Hydrochloric acid: HCl (stomach acid, industry)
- Sulphuric acid: H₂SO₄ (car batteries, fertilisers)
- Nitric acid: HNO₃ (fertilisers, explosives)
- Acetic acid: CH₃COOH (vinegar)
- Citric acid: found in citrus fruits
- Carbonic acid: H₂CO₃ (soft drinks)
2. **Common bases/alkalis:**
- Sodium hydroxide: NaOH (caustic soda, soap making)
- Potassium hydroxide: KOH (liquid soap, batteries)
- Calcium hydroxide: Ca(OH)₂ (slaked lime, whitewash)
- Magnesium hydroxide: Mg(OH)₂ (milk of magnesia, antacid)
- Ammonium hydroxide: NH₄OH (cleaning agents)
3. **pH values to remember:**
- Gastric juice: ~1–2 (strongly acidic)
- Lemon juice: ~2
- Vinegar: ~3
- Pure water: 7 (neutral)
- Blood: ~7.4 (slightly alkaline)
- Baking soda solution: ~8–9
- Soap solution: ~10
- Lime water: ~12
4. **Common salts and uses:**
- Sodium chloride (NaCl): table salt, preservative
- Sodium bicarbonate (NaHCO₃): baking soda, antacid, fire extinguishers
- Sodium carbonate (Na₂CO₃·10H₂O): washing soda, water softener
- Calcium carbonate (CaCO₃): limestone, marble, antacid
- Calcium sulphate (CaSO₄·½H₂O): plaster of Paris
- Copper sulphate (CuSO₄·5H₂O): fungicide, electroplating
- Sodium hydroxide (NaOH): caustic soda, though technically a base
5. **Neutralisation equation template:** Acid + Base → Salt + Water Example: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
6. **Indicator colour changes:**
- Blue litmus turns red in acid, red litmus turns blue in base
- Phenolphthalein: colourless in acid, pink in base
- Methyl orange: red in acid, yellow in base
Worked Examples
**Example 1:** What is the pH of a solution that turns red litmus blue? **Solution:** Red litmus turning blue indicates a basic solution. Bases have pH > 7. So the pH must be greater than 7 (anywhere from 8 to 14).
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**Example 2:** Write the neutralisation reaction when hydrochloric acid reacts with sodium hydroxide. Name the salt formed. **Solution:** Acid: HCl Base: NaOH Reaction: HCl + NaOH → NaCl + H₂O Salt formed: Sodium chloride (common salt). This is a typical acid-base neutralisation producing a neutral salt and water.
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**Example 3:** A student mixes baking soda (sodium bicarbonate, NaHCO₃) with vinegar (acetic acid, CH₃COOH). What happens? **Solution:** Baking soda is a weak base; vinegar is a weak acid. They react: NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂↑ Carbon dioxide gas is released (fizzing/effervescence). This is a neutralisation that also liberates CO₂ because bicarbonate is involved.
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**Example 4:** Why does bee sting cause pain, and how can baking soda help? **Solution:** Bee venom contains formic acid (acidic). Baking soda (NaHCO₃) is a mild base. Applying baking soda neutralises the acid, reducing pain. This is neutralisation in a practical context.
Common Mistakes
1. **Confusing concentration with strength.** Wrong: "Concentrated vinegar is a strong acid." Fix: Vinegar is always a weak acid (acetic acid ionises partially), regardless of concentration. Strength depends on ionisation, concentration is about amount dissolved.
2. **Mixing up indicator colours.** Wrong: "Phenolphthalein turns pink in acid." Fix: Phenolphthalein is colourless in acid and pink in base. Blue litmus turns red in acid; red litmus turns blue in base. Use mnemonics or make a colour table.
3. **Forgetting water of crystallisation in salt formulas.** Wrong: Writing washing soda as Na₂CO₃ only. Fix: Washing soda is Na₂CO₃·10H₂O (10 water molecules). Plaster of Paris is CaSO₄·½H₂O. Always include water of crystallisation when stated.
4. **Assuming all salts are neutral.** Wrong: "Every salt has pH 7." Fix: Salt of strong acid + strong base is neutral (NaCl, pH~7). Salt of weak acid + strong base is basic (Na₂CO₃, pH > 7). Salt of strong acid + weak base is acidic (NH₄Cl, pH < 7).
5. **Not recognising neutralisation in daily life.** Wrong: Treating neutralisation as a lab-only concept. Fix: Antacids (Mg(OH)₂) neutralise stomach acid (HCl). Lime water neutralises acidic soil. Toothpaste neutralises mouth acidity. Linking theory to life aids recall.
Quick Reference
- **pH < 7 = acidic; pH = 7 = neutral; pH > 7 = basic/alkaline.**
- **Neutralisation:** Acid + Base → Salt + Water (exothermic).
- **Blue litmus → red in acid; red litmus → blue in base.**
- **Phenolphthalein:** colourless in acid, pink in base.
- **Common salts:** NaCl (table salt), NaHCO₃ (baking soda), Na₂CO₃ (washing soda), CaCO₃ (limestone), CaSO₄·½H₂O (plaster of Paris).
- **Strong acids:** HCl, H₂SO₄, HNO₃. **Weak acids:** CH₃COOH, citric acid.
- **Strong bases:** NaOH, KOH. **Weak bases:** NH₄OH, Mg(OH)₂.
- **Water of crystallisation:** water molecules in crystal lattice (e.g. CuSO₄·5H₂O is blue; anhydrous CuSO₄ is white).
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**Exam tip:** Revise common salt formulas and their uses the night before. Questions often ask "which salt is used in X?" or "what is the pH of Y?" Quick recall wins marks.