Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chapter in Chemistry for TS TET Paper II, connecting everyday substances like lemon juice, soap and table salt to core chemical principles. This topic tests both conceptual understanding and practical applications—expect questions on identifying acids/bases using indicators, writing neutralisation reactions, understanding pH scale, and recognising common acids, bases and salts with their uses.

For the TET exam, you must master three areas: (1) definitions and properties that distinguish acids from bases, (2) the indicator colour changes that appear frequently in MCQs, and (3) the formation, types and everyday applications of salts. This chapter also builds the foundation for understanding chemical reactions, so a strong grasp here helps across Chemistry topics.

Key Concepts

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

• **Bases** are substances that release hydroxide ions (OH⁻) in water, taste bitter, feel soapy, and turn red litmus blue. Examples: NaOH, KOH, Ca(OH)₂, NH₄OH.

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis (e.g., Cu(OH)₂ is a base but not an alkali as it is insoluble).

• **Neutralisation** is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water. This reaction is exothermic.

• **pH Scale** measures acidity or basicity on a scale of 0 to 14. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Lower pH means stronger acid.

• **Indicators** are substances that show different colours in acidic and basic solutions. They help identify the nature of unknown solutions.

• **Salts** are ionic compounds formed by the reaction of acids and bases. The metal/positive ion comes from the base, and the negative ion comes from the acid.

• **Strong vs Weak Acids/Bases**: Strong acids (HCl, H₂SO₄) completely dissociate in water; weak acids (CH₃COOH, H₂CO₃) partially dissociate. Similarly for bases.

Formulas / Key Facts

| Fact | Detail | |------|--------| | Litmus colour change | Blue litmus → Red in acid; Red litmus → Blue in base | | Phenolphthalein | Colourless in acid; Pink in base | | Methyl orange | Red in acid; Yellow in base | | Turmeric indicator | Yellow in acid/neutral; Reddish-brown in base | | pH of pure water | 7 (neutral) | | pH of gastric juice | About 1.2 (strongly acidic) | | pH of blood | 7.35–7.45 (slightly basic) | | Neutralisation equation | HCl + NaOH → NaCl + H₂O | | Baking soda formula | NaHCO₃ (Sodium hydrogen carbonate) | | Washing soda formula | Na₂CO₃·10H₂O (Sodium carbonate decahydrate) | | Plaster of Paris | CaSO₄·½H₂O (Calcium sulphate hemihydrate) |

**Common Acids and Uses:**

• Hydrochloric acid (HCl) — cleaning metals, stomach acid
• Sulphuric acid (H₂SO₄) — batteries, fertilisers (called "king of chemicals")
• Acetic acid (CH₃COOH) — vinegar, food preservation
• Citric acid — found in citrus fruits, food flavouring

**Common Bases and Uses:**

• Sodium hydroxide (NaOH) — soap making, drain cleaner
• Calcium hydroxide Ca(OH)₂ — whitewashing, neutralising acidic soil
• Magnesium hydroxide Mg(OH)₂ — antacid (milk of magnesia)
• Ammonium hydroxide (NH₄OH) — cleaning agents, fertilisers

Worked Examples

**Example 1: Identifying Acid or Base**

*Problem:* A solution turns blue litmus red and has pH = 3. What is the nature of the solution?

*Solution:*

• Blue litmus turning red indicates an acid
• pH = 3 is less than 7, confirming acidic nature
• Answer: The solution is acidic (and strongly acidic since pH is much below 7)

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**Example 2: Neutralisation Reaction**

*Problem:* Write the balanced equation when sulphuric acid reacts with sodium hydroxide.

*Solution:*

• Acid: H₂SO₄ (provides SO₄²⁻)
• Base: NaOH (provides Na⁺)
• Products: Sodium sulphate + Water
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• Note: 2 NaOH needed because H₂SO₄ is dibasic (has 2 H⁺)

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**Example 3: Indicator-Based Question**

*Problem:* A student adds phenolphthalein to Solution A (turns pink) and Solution B (remains colourless). What can you conclude?

*Solution:*

• Phenolphthalein is pink in basic solution, colourless in acidic/neutral
• Solution A is basic (pH > 7)
• Solution B is acidic or neutral (pH ≤ 7)

Common Mistakes

• **Confusing "all bases are alkalis"** → Correction: Only water-soluble bases are alkalis. Insoluble bases like Fe(OH)₃ or Cu(OH)₂ are not alkalis.

• **Reversing litmus colour changes** → Correction: Remember "BRAT" — Blue litmus turns Red in Acid. Red litmus turns blue in base (opposite).

• **Thinking pH 0 means no acidity** → Correction: pH 0 means maximum acidity. Lower pH = stronger acid. pH 14 = strongest base.

• **Writing wrong products in neutralisation** → Correction: Salt name comes from metal of base + acid radical. NaOH + HCl gives sodium (from NaOH) + chloride (from HCl) = NaCl.

• **Assuming all salts are neutral** → Correction: Salts can be acidic (NH₄Cl), basic (Na₂CO₃), or neutral (NaCl) depending on parent acid and base strength.

• **Forgetting water of crystallisation** → Correction: Washing soda is Na₂CO₃·10H₂O (with 10 water molecules), not just Na₂CO₃. This distinction appears in exams.

Quick Reference

• Acids give H⁺ ions; Bases give OH⁻ ions in water
• pH scale: 0–6 acidic, 7 neutral, 8–14 basic
• Blue litmus → Red = Acid; Red litmus → Blue = Base
• Neutralisation: Acid + Base → Salt + Water (exothermic)
• Baking soda = NaHCO₃; Washing soda = Na₂CO₃·10H₂O
• Strong acid + Strong base = Neutral salt (e.g., NaCl)