Properties, reactions, pH scale, salts and their preparation
An acid is a substance that produces H⁺ (hydrogen/proton) ions when dissolved in water.
Arrhenius definition: Acid → H⁺ + anion (in water)
Example: HCl → H⁺ + Cl⁻ | H₂SO₄ → 2H⁺ + SO₄²⁻
• Sour taste (like lemon juice, vinegar) — never taste in lab!
• Turn blue litmus red
• Have pH less than 7
• React with metals to produce H₂ gas
• React with metal carbonates to produce CO₂
• React with bases to form salt and water (neutralisation)
| Common Acids | Formula | Found in |
|---|---|---|
| Hydrochloric acid | HCl | Stomach (gastric acid) |
| Sulphuric acid | H₂SO₄ | Car batteries, fertilisers |
| Nitric acid | HNO₃ | Fertilisers, explosives |
| Acetic acid | CH₃COOH | Vinegar |
| Citric acid | C₆H₈O₇ | Lemon, oranges |
| Carbonic acid | H₂CO₃ | Fizzy drinks (CO₂ in water) |
Base: A substance that produces OH⁻ (hydroxide) ions in water OR accepts H⁺ ions.
Alkali: A base that is soluble in water (e.g., NaOH, KOH, Ca(OH)₂)
Note: All alkalis are bases, but not all bases are alkalis.
• Bitter taste and soapy/slippery feel — never taste or touch in lab!
• Turn red litmus blue
• Have pH greater than 7
• React with acids to form salt + water
• Aqueous solution conducts electricity
| Common Bases | Formula | Uses |
|---|---|---|
| Sodium hydroxide (Caustic soda) | NaOH | Soap making, paper, drain cleaners |
| Calcium hydroxide (Slaked lime) | Ca(OH)₂ | Whitewashing, water treatment |
| Potassium hydroxide | KOH | Liquid soaps |
| Magnesium hydroxide | Mg(OH)₂ | Milk of magnesia (antacid) |
| Ammonium hydroxide | NH₄OH | Cleaning agent, fertilisers |
| Indicator | In Acid | In Base | In Neutral |
|---|---|---|---|
| Litmus | Red | Blue | Purple |
| Phenolphthalein | Colourless | Pink | Colourless |
| Methyl Orange | Red | Yellow | Orange |
| Turmeric | Yellow | Red-brown | Yellow |
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Fe + 2HCl → FeCl₂ + H₂↑
Mg + 2HCl → MgCl₂ + H₂↑
Note: H₂ gas burns with a 'pop' sound — test for hydrogen
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
CO₂ turns lime water milky — test for CO₂
NaOH + HCl → NaCl + H₂O
Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
KOH + HNO₃ → KNO₃ + H₂O
pH measures the concentration of H⁺ ions in solution. Scale runs from 0 to 14.
• pH < 7 → Acidic (lower pH = stronger acid)
• pH = 7 → Neutral (pure water at 25°C)
• pH > 7 → Basic/Alkaline (higher pH = stronger base)
• Gastric acid (stomach): pH 1–2 (strong acid, digests food)
• Lemon juice: pH 2.5
• Tomato juice: pH 4
• Black coffee: pH 5
• Pure water: pH 7
• Blood: pH 7.4 (slightly basic — must stay in this range!)
• Baking soda: pH 8
• Bleach: pH 12–13
| Salt | Chemical Name | Formula | Uses |
|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Food, preservation, manufacture of NaOH, HCl, Na₂CO₃ |
| Washing soda | Sodium carbonate | Na₂CO₃·10H₂O | Cleaning agent, glass, soap making |
| Baking soda | Sodium bicarbonate | NaHCO₃ | Baking, antacid, fire extinguisher |
| Bleaching powder | Calcium hypochlorite | Ca(OCl)Cl | Disinfectant, bleaching agent, water treatment |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Plaster casts (medical), sculpture |