Acids, Bases and Salts
Overview
Acids, Bases and Salts form a cornerstone chapter in Chemistry for AP TET Paper II. This topic connects everyday substances (vinegar, soap, baking soda) to fundamental chemical concepts, making it essential for both content knowledge and classroom teaching. Questions typically test identification of acids/bases, indicator colour changes, neutralisation reactions, and practical applications.
For upper primary teaching (Classes 6-8), this topic builds scientific vocabulary and introduces students to chemical reactions they observe daily. Mastery requires knowing definitions, properties, common examples, indicator behaviour, and the formation and uses of salts.
Key Concepts
- Acids are substances that release hydrogen ions (H⁺) in water, taste sour, and turn blue litmus red. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), citric acid, 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 (slaked lime), ammonia solution.
- Alkalis are bases that dissolve in water. All alkalis are bases, but not all bases are alkalis. Example: NaOH is an alkali; copper hydroxide is a base but not an alkali (insoluble).
- Indicators are substances that show different colours in acidic and basic solutions. They help identify the nature of a substance 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 by neutralisation. The metal comes from the base; the non-metal part comes from the acid.
- pH Scale measures acidity or basicity on a scale of 0-14. pH 7 is neutral; below 7 is acidic; above 7 is basic. Lower pH means stronger acid; higher pH means stronger base.
Formulas / Key Facts
| Fact | Detail |
|---|---|
| Acid definition | Produces H⁺ ions in water |
| Base definition | Produces OH⁻ ions in water |
| Neutralisation equation | HCl + NaOH → NaCl + H₂O |
| pH of pure water | 7 (neutral) |
| pH of lemon juice | About 2 (acidic) |
| pH of soap solution | About 9-10 (basic) |
| Litmus source | Lichen plant |
| Universal indicator | Shows different colours for different pH values |
Common Acids and Sources:
- Hydrochloric acid (HCl) — stomach, laboratory
- Sulphuric acid (H₂SO₄) — car batteries, fertilisers
- Acetic acid (CH₃COOH) — vinegar
- Citric acid — citrus fruits (lemon, orange)
- Lactic acid — curd, sour milk
- Formic acid — ant sting, nettle sting
- Tartaric acid — tamarind, grapes
Common Bases and Sources:
- Sodium hydroxide (NaOH) — caustic soda, soap making
- Calcium hydroxide Ca(OH)₂ — slaked lime, whitewashing
- Magnesium hydroxide Mg(OH)₂ — milk of magnesia (antacid)
- Ammonium hydroxide (NH₄OH) — cleaning agents
Important Salts:
| Salt | Chemical Formula | Common Name | Use |
|---|---|---|---|
| Sodium chloride | NaCl | Common salt | Cooking, preservation |
| Sodium bicarbonate | NaHCO₃ | Baking soda | Baking, antacid |
| Sodium carbonate | Na₂CO₃ | Washing soda | Cleaning, glass making |
| Calcium sulphate | CaSO₄·½H₂O | Plaster of Paris | Casts, statues |
| Potassium nitrate | KNO₃ | Saltpetre | Fertiliser, gunpowder |
Worked Examples
Example 1: Indicator Colour Change
Question: What colour will turmeric indicator show when added to soap solution?
Solution:
- Step 1: Identify the nature of soap solution → Soap is basic (pH 9-10)
- Step 2: Recall turmeric behaviour → Turmeric turns red/brown in bases, remains yellow in acids
- Step 3: Answer → Turmeric turns reddish-brown in soap solution
Example 2: Neutralisation Reaction
Question: Write the products when sulphuric acid reacts with sodium hydroxide.
Solution:
- Step 1: Write the reactants → H₂SO₄ + NaOH
- Step 2: Apply neutralisation rule → Acid + Base → Salt + Water
- Step 3: Identify the salt → Sodium from base + Sulphate from acid = Sodium sulphate
- Step 4: Balance the equation → H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
- Answer: Sodium sulphate and water
Example 3: pH Application
Question: A farmer finds his soil has pH 4. What should he add to make it suitable for crops?
Solution:
- Step 1: Analyse pH 4 → Highly acidic soil
- Step 2: To neutralise acid, add a base
- Step 3: Common agricultural base → Slaked lime (calcium hydroxide) or quicklime (calcium oxide)
- Answer: The farmer should add slaked lime or quicklime to neutralise the acidic soil
Common Mistakes
- Confusing acids with bases based on formula → Students see OH in acetic acid (CH₃COOH) and think it's a base. Correct thinking: Check if the substance releases H⁺ (acid) or OH⁻ (base) in water. Acetic acid releases H⁺, so it's an acid.
- Assuming all salts are neutral → Students believe every salt has pH 7. Correct thinking: Salts from strong acid + weak base are acidic (NH₄Cl); salts from weak acid + strong base are basic (Na₂CO₃). Only salts from strong acid + strong base are neutral (NaCl).
- Mixing up litmus colour changes → "Blue to red" vs "red to blue" confusion. Memory aid: Blue litmus turns Red in Acid (BRA). Red litmus turns blue in base.
- Thinking neutralisation needs equal volumes → Students assume 10 mL acid + 10 mL base always neutralises completely. Correct thinking: Neutralisation depends on concentration and molarity, not just volume.
- Forgetting water as a neutralisation product → Writing only salt as the product. Correct thinking: Always write Acid + Base → Salt + Water. The water formation is what makes the reaction exothermic.
Quick Reference
- Acids: Sour, release H⁺, turn blue litmus red, pH < 7
- Bases: Bitter, soapy, release OH⁻, turn red litmus blue, pH > 7
- Litmus colours: Acid = Red, Base = Blue (remember "Acid-Red")
- Neutralisation: Acid + Base → Salt + Water (always exothermic)
- Antacids work by: Neutralising excess stomach acid (Mg(OH)₂ in milk of magnesia)
- Baking soda (NaHCO₃): Used in cooking, fire extinguishers, and as antacid