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Acids, Bases & Neutralisation

pH, Indicators & Reactions

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Learning Objectives

Define acids and bases

Understand the pH scale

Use indicators to identify pH

Distinguish strong and weak acids/bases

Write equations for acid reactions

Understand neutralisation

Acids Defined

Acid: Substance that produces H⁺ (hydrogen) ions in aqueous solution.

Examples: HCl, H₂SO₄, HNO₃ (strong acids)
Examples: CH₃COOH, H₂CO₃ (weak acids)
Taste sour, turn litmus red, have pH < 7

Bases Defined

Base: Substance that produces OH⁻ (hydroxide) ions in aqueous solution.

Alkali: Soluble base in water
Examples: NaOH, KOH, Ca(OH)₂, NH₃
Feel soapy, turn litmus blue, have pH > 7

The pH Scale

pH: Measure of acidity/alkalinity (scale 0-14 at 25°C)

pH 0-6: Acidic
pH 7: Neutral
pH 8-14: Alkaline (basic)
Each unit change = 10× change in H⁺ concentration

pH Scale Examples

pH 0-1: Very strong acids

pH 2: Lemon juice, vinegar

pH 7: Pure water, neutral

pH 11-12: Ammonia solution, bleach

pH 13-14: Strong alkalis

Indicators

Indicator: Substance that changes color depending on pH.

Litmus: Red (acidic) ↔ Blue (alkaline)
Methyl orange: Red (acidic) ↔ Yellow (neutral/alkaline)
Phenolphthalein: Colorless (acidic) ↔ Pink (alkaline)

Strong Acids

Strong acid: Completely ionizes in aqueous solution; produces many H⁺ ions.

HCl → H⁺ + Cl⁻ (100% ionization)
H₂SO₄ → 2H⁺ + SO₄²⁻
HNO₃ → H⁺ + NO₃⁻
Lower pH, conducts electricity well

Weak Acids

Weak acid: Partially ionizes in aqueous solution; produces fewer H⁺ ions.

CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partial ionization)
H₂CO₃ ⇌ HCO₃⁻ + H⁺
Higher pH, conducts electricity poorly

Acid Reactions

Acid + Metal → Salt + Hydrogen: 2HCl + Zn → ZnCl₂ + H₂

Acid + Carbonate → Salt + CO₂ + Water: 2HCl + CaCO₃ → CaCl₂ + CO₂ + H₂O

Acid + Base → Salt + Water (neutralisation)

Neutralisation

Neutralisation: Reaction between acid and base to produce salt and water.

HCl + NaOH → NaCl + H₂O
H⁺ + OH⁻ → H₂O (ionic equation for all neutralisations)

Salt Formation

Salt: Ionic compound formed from acid and base.

Acid name + Base name → Salt name
Hydrochloric acid + Sodium hydroxide → Sodium chloride
Sulfuric acid + Potassium hydroxide → Potassium sulfate

Strong vs Weak Bases

Strong base: NaOH, KOH, Ca(OH)₂ (completely ionize)

Weak base: NH₃ (partially ionizes)

Strong base has pH 13-14

Weak base has pH 8-11

Volumetric Analysis

Using a known volume and concentration of one substance to find another's concentration.

Titration: Add acid to alkali (or vice versa) until indicator changes
At equivalence point: moles of acid = moles of base
Use: n = cV to calculate concentrations

Lesson Summary

Acids produce H⁺ ions; bases produce OH⁻ ions

pH scale: 0-6 acidic, 7 neutral, 8-14 alkaline

Indicators change color with pH

Strong acids/bases completely ionize; weak partially

Neutralisation: Acid + Base → Salt + Water

Ionic equation: H⁺ + OH⁻ → H₂O (for all neutralisations)