Properties, Indicators and Neutralisation
Acids, Bases and Salts: Properties, Indicators and Neutralisation
Properties, Indicators and Neutralisation
Acids, Bases and Salts
What you'll learn
- What acids and bases are; their properties.
- How indicators tell us if something is acidic or basic.
- The pH scale — what it means.
- Neutralisation reaction and its everyday uses.
- Common salts and how they form.
Key concepts
Acids
- Acid: a substance that tastes sour, turns blue litmus red, and releases H⁺ ions in water.
- Examples:
| Acid | Chemical formula | Found in / used for |
|---|---|---|
| Hydrochloric acid | HCl | Stomach (gastric acid); cleaning metals |
| Sulphuric acid | H₂SO₄ | Car batteries; industrial processes |
| Nitric acid | HNO₃ | Fertilisers; explosives |
| Acetic acid | CH₃COOH | Vinegar (5% solution) |
| Citric acid | C₆H₈O₇ | Lemons, oranges, tamarind |
| Tartaric acid | C₄H₆O₆ | Grapes, baking powder |
| Lactic acid | C₃H₆O₃ | Curd, sour milk |
| Formic acid | HCOOH | Ant sting, nettle sting |
- Properties of acids:
- Sour taste (never taste in lab!).
- Turn blue litmus red.
- React with metals → hydrogen gas + salt. (Zn + H₂SO₄ → ZnSO₄ + H₂↑)
- React with carbonates → CO₂ gas + water + salt.
- Conduct electricity (in solution) — ionic.
Bases
- Base: a substance that tastes bitter, feels slippery/soapy, turns red litmus blue.
- Alkali: a base that dissolves in water (all alkalis are bases; not all bases are alkalis).
- Examples:
| Base | Chemical formula | Found in / used for |
|---|---|---|
| Sodium hydroxide (caustic soda) | NaOH | Making soap; paper |
| Potassium hydroxide | KOH | Making soap |
| Calcium hydroxide (slaked lime) | Ca(OH)₂ | White-washing; water treatment |
| Magnesium hydroxide | Mg(OH)₂ | Milk of Magnesia (antacid) |
| Ammonia solution | NH₄OH | Cleaning agents; fertilisers |
| Baking soda | NaHCO₃ | Cooking; fire extinguishers |
- Properties of bases:
- Bitter taste; soapy/slippery feel.
- Turn red litmus blue.
- React with acids → neutralisation.
- Conduct electricity (in solution).
Indicators
An indicator changes colour depending on whether a substance is acidic or basic.
| Indicator | In acid | In base | In neutral |
|---|---|---|---|
| Litmus | Red | Blue | Purple |
| Turmeric | Yellow | Red-brown | Yellow |
| China rose (Hibiscus) | Dark pink/magenta | Green | Pink |
| Red cabbage juice | Pink/red | Green/yellow | Purple |
| Phenolphthalein | Colourless | Pink | Colourless |
| Methyl orange | Red | Yellow | Orange |
- Natural indicators: litmus (from lichen), turmeric, china rose, red cabbage.
- Litmus paper: blue litmus turns red in acid; red litmus turns blue in base.
- Olfactory indicators: substances whose smell changes — onion (smell disappears in base), vanilla (smell disappears in base).
The pH Scale
- pH: measure of acidity or alkalinity; scale from 0 to 14.
- pH 7 = neutral (pure water).
- pH < 7 = acidic (lower = more acidic).
- pH > 7 = basic/alkaline (higher = more basic).
| pH | Example |
|---|---|
| 0 | Battery acid (H₂SO₄ concentrated) |
| 2 | Stomach acid (HCl) |
| 3 | Vinegar, lemon juice |
| 4–5 | Acid rain (< 5.6 is acid rain) |
| 6–7 | Milk, pure water |
| 7.4 | Human blood |
| 8–9 | Baking soda, sea water |
| 11 | Ammonia |
| 14 | NaOH (concentrated) |
- Universal indicator: mixture of indicators that shows a range of colours for different pH values.
Neutralisation
- Neutralisation: reaction between an acid and a base to form salt + water.
Acid + Base → Salt + Water
- The solution becomes neutral (or close to neutral) — pH ≈ 7.
- Exothermic reaction (releases heat).
Examples:
- HCl + NaOH → NaCl + H₂O (common salt + water)
- H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2H₂O
- HNO₃ + KOH → KNO₃ + H₂O
Salts
- Salt: ionic compound formed from neutralisation of acid and base.
- Named: [metal from base] [non-metal/acid radical from acid].
| Acid | Base | Salt formed |
|---|---|---|
| Hydrochloric acid | Sodium hydroxide | Sodium chloride (NaCl) — table salt |
| Sulphuric acid | Calcium hydroxide | Calcium sulphate (CaSO₄) — plaster of Paris raw material |
| Nitric acid | Potassium hydroxide | Potassium nitrate (KNO₃) — fertiliser, gunpowder |
| Carbonic acid | Sodium hydroxide | Sodium carbonate (Na₂CO₃) — washing soda |
Everyday applications of neutralisation
| Situation | Problem | Solution |
|---|---|---|
| Ant/wasp sting | Formic acid injected | Apply baking soda (base) — neutralises acid |
| Bee sting | Alkaline sting | Apply vinegar/lemon juice (acid) |
| Acidity/heartburn | Excess HCl in stomach | Antacid tablet (Mg(OH)₂ or NaHCO₃) — neutralises |
| Soil too acidic | Low pH hurts crops | Add lime (CaO) or slaked lime — raises pH |
| Soil too alkaline | High pH hurts crops | Add acidic fertilisers or organic matter |
| Factory effluents | Acidic waste water | Treat with lime before releasing |
Quick check
- Name two acids found naturally in food and what they're found in.
- What is the difference between a base and an alkali?
- What colour does turmeric turn in a base?
- What is neutralisation? Write a word equation.
- A bee sting is alkaline. Should you apply vinegar or baking soda? Why?
Open the Practice tab for graded questions on Acids, Bases and Salts.
Key Takeaways (TL;DR)
- What you'll learn
- Key concepts
- Quick check
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