Learning Objectives
- Understand the chemical properties of acids and bases
- Classify substances as acidic, basic, or neutral using indicators
- Understand the pH scale and its importance
- Learn about common salts and their preparation
Key Concepts
Acids and Bases
Acids produce H⁺ (H₃O⁺) ions in aqueous solution. Examples: HCl, H₂SO₄, HNO₃, CH₃COOH.
Bases produce OH⁻ ions in aqueous solution. Examples: NaOH, KOH, Ca(OH)₂.
Indicators
- Litmus: Red in acid, blue in base.
- Methyl orange: Red/pink in acid, yellow in base.
- Phenolphthalein: Colourless in acid, pink in base.
- Olfactory indicators: Onion and vanilla change smell in acid/base.
Chemical Properties
Acid + Metal → Salt + Hydrogen gas: 2HCl + Zn → ZnCl₂ + H₂↑
Base + Metal → Salt + Hydrogen gas: 2NaOH + Zn → Na₂ZnO₂ + H₂↑
Acid + Metal carbonate → Salt + Water + CO₂: 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑
Acid + Base → Salt + Water (Neutralisation reaction)
The pH Scale
pH is a measure of hydrogen ion concentration. Scale ranges from 0 to 14.
- pH < 7: Acidic solution (lower pH = stronger acid)
- pH = 7: Neutral solution
- pH > 7: Basic/alkaline solution (higher pH = stronger base)
Our body works within a narrow pH range. Stomach: pH ~1.2 (HCl); Blood: pH 7.35-7.45; soil pH affects plant growth.
Important Salts
Common salt (NaCl): Raw material for NaOH, baking soda, washing soda, and bleaching powder.
Baking soda (NaHCO₃): Used in baking, antacids, fire extinguishers.
Washing soda (Na₂CO₃·10H₂O): Used in cleaning, water softening, glass manufacture.
Plaster of Paris (CaSO₄·½H₂O): Used in casts, moulds, and decoration. Sets to hard mass (gypsum) on adding water.
Bleaching powder (CaOCl₂): Used for water purification, disinfection, and bleaching.
Summary
Acids donate H⁺ ions and bases donate OH⁻ ions in solution. The pH scale quantifies acidity or basicity. Neutralisation produces salt and water. Several important salts are manufactured from common salt (NaCl) and have wide applications in industry and daily life.
Important Terms
- Neutralisation
- Reaction of an acid with a base to form salt and water
- pH Scale
- A logarithmic scale from 0 to 14 measuring the acidity or basicity of a solution
- Water of Crystallisation
- Fixed number of water molecules present in one formula unit of a salt (e.g., CuSO₄·5H₂O)
Quick Revision
- Acids produce H⁺ ions; Bases produce OH⁻ ions in water
- pH < 7: acidic; pH = 7: neutral; pH > 7: basic
- Acid + Base → Salt + Water (neutralisation)
- Baking soda: NaHCO₃; Washing soda: Na₂CO₃·10H₂O
- Plaster of Paris hardens by absorbing water to form gypsum (CaSO₄·2H₂O)