Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry topic in TN TET Paper II, appearing consistently in the Mathematics and Science section. This topic connects directly to everyday life—from the citric acid in lemons to the sodium hydroxide in soap—making it both practically relevant and conceptually important for upper primary teaching.

For TN TET, expect questions on identification of acids and bases using indicators, properties and chemical reactions, pH scale interpretation, and common salts with their uses. The pedagogy aspect often asks how to demonstrate these concepts through simple classroom experiments. Mastering this topic requires understanding the characteristic properties, memorising key examples, and knowing the neutralisation reaction thoroughly.

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: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), acetic acid (vinegar).

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

• **Alkalis** are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis. Example: NaOH is an alkali; Cu(OH)₂ is a base but not an alkali (insoluble).

• **Indicators** are substances that show different colours in acidic and basic solutions. They help identify whether a substance is acid or base without tasting.

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

• **Salts** are ionic compounds formed from neutralisation reactions. They contain a metal (from base) and a non-metal part (from acid).

• **pH Scale** measures acidity or basicity on a scale of 0 to 14. pH 7 is neutral; below 7 is acidic; above 7 is basic. Lower pH means stronger acid; higher pH means stronger base.

• **Strong vs Weak acids/bases**: Strong acids (HCl, H₂SO₄) ionise completely; weak acids (acetic acid, citric acid) ionise partially. Same distinction applies to bases.

Formulas / Key Facts

**Essential Reactions:**

• Acid + Metal → Salt + Hydrogen gas

Example: 2HCl + Zn → ZnCl₂ + H₂↑

• Acid + Metal carbonate → Salt + Water + Carbon dioxide

Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑

• Acid + Metal bicarbonate → Salt + Water + Carbon dioxide

Example: HCl + NaHCO₃ → NaCl + H₂O + CO₂↑

• Acid + Base → Salt + Water (Neutralisation)

Example: HCl + NaOH → NaCl + H₂O

• Acid + Metal oxide → Salt + Water

Example: 2HCl + CuO → CuCl₂ + H₂O

**Indicator Colour Chart:**

| Indicator | In Acid | In Base | Neutral | |-----------|---------|---------|---------| | Litmus | Red | Blue | Purple | | Phenolphthalein | Colourless | Pink | Colourless | | Methyl orange | Red | Yellow | Orange | | Turmeric | Yellow | Reddish-brown | Yellow |

**Common Salts and Uses:**

• Sodium chloride (NaCl) — Table salt, food preservation
• Sodium bicarbonate (NaHCO₃) — Baking soda, antacid
• Sodium carbonate (Na₂CO₃) — Washing soda, glass making
• Calcium sulphate (CaSO₄·½H₂O) — Plaster of Paris, casts
• Potassium nitrate (KNO₃) — Fertiliser, gunpowder
• Bleaching powder (CaOCl₂) — Disinfection, bleaching

**pH Values to Remember:**

• Gastric juice: 1-2 (strongly acidic)
• Lemon juice: 2.2
• Vinegar: 2.5
• Pure water: 7 (neutral)
• Blood: 7.4 (slightly basic)
• Milk of magnesia: 10
• Soap solution: 9-10

Worked Examples

**Example 1: Identifying Products of Neutralisation**

*Question:* What happens when dilute hydrochloric acid is added to sodium hydroxide solution? Write the balanced equation.

*Solution:*

• HCl is an acid; NaOH is a base
• This is a neutralisation reaction
• Products are salt (sodium chloride) and water
• Balanced equation: HCl + NaOH → NaCl + H₂O
• The reaction is exothermic (releases heat)

**Example 2: Predicting Indicator Behaviour**

*Question:* A solution turns phenolphthalein pink and blue litmus remains blue. What is the nature of the solution?

*Solution:*

• Phenolphthalein turns pink only in basic solutions
• Blue litmus remains blue in basic solutions (would turn red in acid)
• Both observations confirm: the solution is basic (pH > 7)

**Example 3: Salt Formation**

*Question:* Name the acid and base needed to prepare copper sulphate. Write the reaction.

*Solution:*

• Copper sulphate (CuSO₄) contains copper (Cu) and sulphate (SO₄)
• Sulphate comes from sulphuric acid (H₂SO₄)
• Copper comes from copper oxide or copper hydroxide
• Reaction: H₂SO₄ + CuO → CuSO₄ + H₂O
• Or: H₂SO₄ + Cu(OH)₂ → CuSO₄ + 2H₂O

Common Mistakes

• **Confusing alkalis with all bases** → Remember: alkalis are water-soluble bases only. Copper hydroxide is a base but not an alkali because it does not dissolve in water.

• **Thinking neutral pH means no reaction** → A salt solution can be neutral (pH 7) but is still chemically different from water. NaCl solution is neutral but contains ions.

• **Assuming all salts are neutral** → Salts of strong acid + weak base are acidic (NH₄Cl, pH < 7). Salts of weak acid + strong base are basic (Na₂CO₃, pH > 7). Only strong acid + strong base gives neutral salt.

• **Forgetting gas evolution tests** → When acid reacts with carbonate, CO₂ is released (turns lime water milky). When acid reacts with metal, H₂ is released (burns with pop sound). These are key identification tests.

• **Mixing up litmus colour changes** → Use the mnemonic "ARAB": Acid turns Red, Alkali turns Blue. Or remember "Blue to Red = Acid (BRA)."

Quick Reference

• Acids give H⁺ ions; bases give OH⁻ ions in water.

• pH scale: 0-14; below 7 acidic, 7 neutral, above 7 basic.

• Neutralisation: Acid + Base → Salt + Water (always exothermic).

• Litmus test: Acid = red, Base = blue.

• Baking soda (NaHCO₃) is used as antacid because it neutralises excess stomach acid.

• Turmeric is a natural indicator—turns reddish-brown in bases (why soap stains on clothes appear red).