IGCSE Chemistry Acids and Bases Explained: Simple Guide

Acids and bases are fundamental concepts in IGCSE Chemistry. This simple guide explains what acids and bases are, how to identify them, pH scale, indicators, and acid-base reactions, with clear explanations and everyday examples.

What are Acids?

Acids are substances that:

• Have a pH less than 7
• Turn blue litmus paper red
• Produce H+ ions in water
• Taste sour (don’t taste in lab!)
• React with metals to produce hydrogen gas

Common Acids:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Ethanoic acid (vinegar, CH₃COOH)

Properties:

• Corrosive
• Conduct electricity (when dissolved)
• React with carbonates to produce CO₂
• React with bases to form salts

What are Bases?

Bases are substances that:

• Have a pH greater than 7
• Turn red litmus paper blue
• Produce OH- ions in water
• Feel slippery
• Neutralize acids

Common Bases:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)₂)
• Ammonia (NH₃)

Alkalis:

• Bases that dissolve in water
• All alkalis are bases
• Not all bases are alkalis
• Examples: NaOH, KOH

The pH Scale

pH Scale:

• Measures acidity/alkalinity
• Range: 0 to 14
• pH 7: neutral (pure water)
• pH < 7: acidic (lower = more acidic)
• pH > 7: alkaline (higher = more alkaline)

pH Values:

• pH 0-2: Strong acids (stomach acid, battery acid)
• pH 3-6: Weak acids (vinegar, orange juice)
• pH 7: Neutral (pure water)
• pH 8-11: Weak alkalis (soap, baking soda)
• pH 12-14: Strong alkalis (drain cleaner, oven cleaner)

Indicators

Universal Indicator

• Shows full pH range
• Changes color across scale
• Red (acid) → Yellow → Green (neutral) → Blue → Purple (alkali)

Litmus Paper

• Red litmus: turns blue in base
• Blue litmus: turns red in acid
• Simple acid/base test

Phenolphthalein

• Colorless in acid
• Pink in alkali
• Used in titrations

Methyl Orange

• Red in acid
• Yellow in alkali
• Used in titrations

Neutralization

What is Neutralization?

• Acid + Base → Salt + Water
• H+ ions + OH- ions → H₂O
• pH moves toward 7
• Heat is usually produced

Examples:

• HCl + NaOH → NaCl + H₂O
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
• CH₃COOH + NaOH → CH₃COONa + H₂O

Uses:

• Indigestion tablets (neutralize stomach acid)
• Soil treatment (lime to reduce acidity)
• Ant stings (baking soda for formic acid)
• Bee stings (vinegar for alkali)

Acid-Base Reactions

Acids and Metals

• Acid + Metal → Salt + Hydrogen
• Example: 2HCl + Zn → ZnCl₂ + H₂
• Not all metals react (reactivity series)
• Test for hydrogen: pop test

Acids and Carbonates

• Acid + Carbonate → Salt + Water + Carbon Dioxide
• Example: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
• Test for CO₂: limewater turns cloudy
• Used in antacid tablets

Acids and Bases

• Acid + Base → Salt + Water
• Neutralization reaction
• Produces salt and water only

Salts

What are Salts?

• Formed when acid reacts with base
• Made of positive and negative ions
• Examples: NaCl, CuSO₄, CaCO₃

Naming Salts:

• From hydrochloric acid: chloride salts
• From sulfuric acid: sulfate salts
• From nitric acid: nitrate salts
• From ethanoic acid: ethanoate salts

Making Salts:

• Acid + metal
• Acid + base
• Acid + carbonate
• Choose method based on salt solubility

Real-World Applications

Everyday Examples:

• Stomach acid: HCl for digestion
• Antacids: bases to neutralize excess acid
• Soap: alkaline, cleans by neutralizing acids
• Battery acid: sulfuric acid

Industrial Uses:

• Fertilizers: nitric acid, phosphoric acid
• Cleaning products: acids and alkalis
• Food preservation: acids prevent spoilage
• Water treatment: pH adjustment

Common Mistakes to Avoid

1. Confusing pH Values

• Lower pH = more acidic (not less acidic)
• pH 1 is more acidic than pH 6
• Remember: lower number = stronger acid

2. Acid + Base ≠ Always Neutral

• Need correct amounts
• Strong acid + weak base ≠ pH 7
• Use indicators to check

3. Not All Bases are Alkalis

• Alkalis dissolve in water
• Bases may not dissolve
• All alkalis are bases, not vice versa

Practice Problems

Problem 1: What is the pH of a strong acid?

• Answer: pH 0-2 (very acidic)

Problem 2: What happens when you add acid to blue litmus?

• Answer: Turns red (acidic)

Problem 3: Complete: HCl + NaOH → ?

• Answer: NaCl + H₂O (salt + water)

Related Resources

• IGCSE Chemistry Revision Notes
• IGCSE Chemistry Past Papers
• IGCSE Chemistry Tutor

---

Understanding acids and bases is essential for IGCSE Chemistry. Practice regularly and seek help when needed to master this fundamental concept.