Acids, Bases and Salts IGCSE Chemistry: Complete Guide with Memory Tips

🧪 Understanding Acids, Bases and Salts in IGCSE Chemistry

Acids, bases, and salts are fundamental concepts in IGCSE Chemistry. This comprehensive guide covers pH, neutralization reactions, and salt preparation with proven memory techniques and practical applications.

📚 Key Theory: Acids and Bases

Acids

Definition: Substances that release H⁺ ions in aqueous solution Properties:

• Sour taste (e.g., citric acid in lemons)
• Turn litmus paper red
• pH less than 7
• React with metals to produce hydrogen gas
• Conduct electricity (ionize in water)

Bases

Definition: Substances that release OH⁻ ions in aqueous solution Properties:

• Bitter taste and soapy feel
• Turn litmus paper blue
• pH greater than 7
• React with acids to form salts and water
• Conduct electricity (ionize in water)

Alkalis

Definition: Bases that are soluble in water Examples: NaOH, KOH, Ca(OH)₂

🧠 Memory Tips and Techniques

1. Acid Properties Memory: “SOUR TURN REACT CONDUCT”

• Sour taste
• Oxidize metals
• Under 7 pH
• Red litmus
• Turn indicators
• Universal indicator changes
• Release H⁺ ions
• Neutralize bases

2. Base Properties Memory: “BITTER BLUE BASIC CONDUCT”

• Bitter taste
• Ionize in water
• Turn litmus blue
• Taste soapy
• Electrical conductors
• Release OH⁻ ions

3. pH Scale Memory: “0-6 Acid, 7 Neutral, 8-14 Base”

• 0-6: Acidic (lower = more acidic)
• 7: Neutral (pure water)
• 8-14: Basic/Alkaline (higher = more basic)

4. Strong vs Weak: “Complete vs Partial”

• Strong: Complete ionization
• Weak: Partial ionization

📖 The pH Scale

pH Definition

• pH: “Power of Hydrogen” or “Potential of Hydrogen”
• Scale: 0 to 14
• Mathematical: pH = -log[H⁺]

pH Values of Common Substances

• Battery acid: pH 0 (very acidic)
• Lemon juice: pH 2 (acidic)
• Coffee: pH 5 (weakly acidic)
• Pure water: pH 7 (neutral)
• Baking soda: pH 9 (weakly basic)
• Household ammonia: pH 11 (basic)
• Bleach: pH 13 (very basic)

Indicators

• Litmus paper: Red (acid), Blue (base)
• Universal indicator: Color range for different pH
• Phenolphthalein: Colorless (acid), Pink (base)
• Methyl orange: Red (acid), Yellow (base)

🎯 IGCSE Exam Focus Areas

Common Exam Questions

1. Identify acids and bases from properties (4 marks)
2. Write neutralization equations (3 marks)
3. Explain pH scale and indicators (5 marks)
4. Describe salt preparation methods (6 marks)

Key Reaction Types

1. Acid + Base → Salt + Water
2. Acid + Metal → Salt + Hydrogen
3. Acid + Carbonate → Salt + Water + Carbon dioxide

⚖️ Neutralization Reactions

General Equation

Acid + Base → Salt + Water

Examples

• HCl + NaOH → NaCl + H₂O
• H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
• HNO₃ + NH₃ → NH₄NO₃

Ionic Equation

H⁺ + OH⁻ → H₂O

Energy Changes

• Neutralization is exothermic: Heat released
• Temperature increases during reaction
• Applications: Heat packs, industrial processes

🧂 Salt Formation

Methods of Salt Preparation

1. Acid + Metal

Suitable for: Reactive metals (not too reactive) Example: Zn + H₂SO₄ → ZnSO₄ + H₂ Process: Add metal to acid until no more reaction

2. Acid + Base/Alkali

Suitable for: Soluble salts Example: HCl + NaOH → NaCl + H₂O Process: Titration to find exact amounts

3. Acid + Carbonate

Suitable for: Most salts Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ Process: Add carbonate until no more fizzing

4. Precipitation

Suitable for: Insoluble salts Example: AgNO₃ + NaCl → AgCl + NaNO₃ Process: Mix solutions, filter precipitate

📊 Common Acids and Bases

Strong Acids

• Hydrochloric acid (HCl): Stomach acid, cleaning
• Sulfuric acid (H₂SO₄): Car batteries, fertilizers
• Nitric acid (HNO₃): Explosives, fertilizers

Weak Acids

• Ethanoic acid (CH₃COOH): Vinegar
• Citric acid: Fruits, food preservative
• Carbonic acid (H₂CO₃): Carbonated drinks

Strong Bases

• Sodium hydroxide (NaOH): Soap making, drain cleaner
• Potassium hydroxide (KOH): Soap, batteries
• Calcium hydroxide (Ca(OH)₂): Lime water, mortar

Weak Bases

• Ammonia (NH₃): Cleaning products, fertilizers
• Sodium hydrogen carbonate (NaHCO₃): Baking soda

🔬 Practical Applications

Industrial Uses

• Sulfuric acid: Most produced chemical worldwide
• Hydrochloric acid: Metal cleaning, pH control
• Sodium hydroxide: Paper making, soap production

Everyday Applications

• Antacids: Neutralize stomach acid
• Toothpaste: Contains mild bases
• Fertilizers: Provide nutrients for plants
• Food preservation: Acids prevent bacterial growth

Environmental Applications

• Acid rain: Environmental problem
• Limestone: Neutralizes acidic soil
• Water treatment: pH adjustment

🧪 Laboratory Techniques

Titration

Purpose: Find concentration of acid or base Process:

1. Measure known volume of solution
2. Add indicator
3. Slowly add titrant until color change
4. Record volume used
5. Calculate concentration

Testing for Gases

• Hydrogen: Squeaky pop with lighted splint
• Carbon dioxide: Turns lime water milky
• Oxygen: Relights glowing splint

Crystallization

Purpose: Obtain pure salt crystals Process:

1. Heat solution to evaporate water
2. Cool to form crystals
3. Filter and wash crystals
4. Dry in warm oven

⚗️ Salt Solubility Rules

Soluble Salts

• All nitrates: NO₃⁻
• All chlorides: Cl⁻ (except AgCl, PbCl₂)
• All sulfates: SO₄²⁻ (except BaSO₄, PbSO₄)
• Group 1 compounds: Na⁺, K⁺, etc.

Insoluble Salts

• Most carbonates: CO₃²⁻ (except Group 1)
• Most hydroxides: OH⁻ (except Group 1, Ca(OH)₂)
• Most sulfides: S²⁻ (except Group 1)

🌡️ Concentration and Dilution

Concentration Units

• Molarity (M): Moles per liter
• g/dm³: Grams per cubic decimeter
• Percentage: Mass/volume percentage

Dilution Calculations

Formula: C₁V₁ = C₂V₂

• C₁ = Initial concentration
• V₁ = Initial volume
• C₂ = Final concentration
• V₂ = Final volume

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