Acids, Bases and the pH Scale
~10 min read
- Acid: Sour, turns blue litmus red, reacts with metals to release H₂. Donates H⁺. Strong: HCl, H₂SO₄, HNO₃.
- Base: Bitter, slippery, turns red litmus blue. Accepts H⁺. Strong: NaOH, KOH, Ca(OH)₂.
- pH: Scale of acidity. pH 0-6 acidic, 7 neutral, 8-14 basic.
Acids and bases are central to chemistry. NDA tests examples, properties, pH scale, neutralisation, and indicators.
Three Definitions
- Arrhenius (1884): Acid releases H⁺ in water; base releases OH⁻.
- Brønsted-Lowry (1923): Acid is a proton (H⁺) donor; base is a proton acceptor. (More general — works without water.)
- Lewis (1923): Acid is an electron-pair acceptor; base is an electron-pair donor. (Most general.)
Common Acids and Bases
| Strong acids | Strong bases |
|---|---|
| HCl (Hydrochloric) | NaOH (Sodium hydroxide / caustic soda) |
| H₂SO₄ (Sulphuric) | KOH (Potassium hydroxide) |
| HNO₃ (Nitric) | Ca(OH)₂ (Calcium hydroxide / slaked lime) |
| HBr, HI, HClO₄ | NaOH and KOH are most common |
| Weak acids | Weak bases |
|---|---|
| CH₃COOH (Acetic / vinegar) | NH₄OH (Ammonium hydroxide) |
| H₂CO₃ (Carbonic — in soda) | Mg(OH)₂ (Milk of magnesia) |
| HCN (Hydrocyanic) |
Properties of Acids and Bases
| Property | Acid | Base |
|---|---|---|
| Taste | Sour | Bitter |
| Touch | — | Soapy/slippery |
| Blue litmus | Turns red | No change |
| Red litmus | No change | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Reaction with metals | Releases H₂ gas | — |
| Reaction with carbonates | Releases CO₂ | — |
pH Scale
- pH = -log₁₀[H⁺]. Scale 0 to 14.
- pH 0-6.9: Acidic. Lower = stronger acid.
- pH 7: Neutral (pure water at 25°C).
- pH 7.1-14: Basic. Higher = stronger base.
- pH values of common substances:
- Battery acid: ~0.
- Lemon juice: 2-3.
- Vinegar: ~3.
- Stomach acid: 1.5-3.5.
- Pure water: 7.
- Blood: 7.35-7.45.
- Sea water: ~8.
- Baking soda solution: 9.
- Soap: 9-10.
- Household ammonia: 11.
- Caustic soda: 13.
Neutralisation
Acid + Base → Salt + Water (exothermic — heat released).
- HCl + NaOH → NaCl + H₂O.
- H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O.
- Useful in: treating acid indigestion (antacids like milk of magnesia), bee sting (apply baking soda), wasp sting (apply vinegar — wasp venom is alkaline).
NDA PYQ Examples
Q: The pH of a neutral solution is:
(a) 0 (b) 7 (c) 10 (d) 14
Answer: (b) 7.
Q: Which is a strong acid?
(a) Acetic acid (b) Carbonic acid (c) Hydrochloric acid (d) Citric acid
Answer: (c) Hydrochloric acid (HCl) — fully ionises in water.
Q: Phenolphthalein in a basic solution is:
(a) Red (b) Pink (c) Yellow (d) Colourless
Answer: (b) Pink (colourless in acid).
Q: Acids react with metals to produce:
(a) Oxygen (b) Hydrogen (c) Chlorine (d) CO₂
Answer: (b) Hydrogen gas.
Q: The pH of blood is approximately:
(a) 5 (b) 7 (c) 7.4 (d) 9
Answer: (c) 7.4 (slightly alkaline; tightly regulated).
Drill Acids, Bases and the pH Scale for NDA
NDA-pattern items on Acids, Bases and the pH Scale with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
What is the difference between a strong acid and a concentrated acid?
Strong = completely ionises in water (HCl, H₂SO₄). Concentrated = has high amount of acid per unit volume. A strong acid can be dilute; a weak acid can be concentrated. Often confused but distinct.
Why does an antacid relieve heartburn?
Stomach acid (HCl, pH ~1.5-3.5) can backflow into oesophagus, causing heartburn. Antacids contain weak bases (milk of magnesia, calcium carbonate) that neutralise excess acid, raising pH.
Why does aluminium foil react with citrus fruit?
Citrus juices are acidic (citric acid, pH ~2-3). Acid reacts with Al producing aluminium ions and hydrogen gas. The reaction may impart metallic taste and tiny perforations in foil.
Why does shampoo say 'pH balanced'?
Hair and skin are slightly acidic (pH ~4.5-5.5). Shampoos formulated near this pH don't disrupt the hair cuticle's structure or skin's protective acid mantle.
Why are some 'natural acids' weak?
Acids like acetic (vinegar), citric (lemon), lactic (milk), oxalic (spinach) only partly ionise in water. They release fewer H⁺ ions than strong mineral acids, giving them higher pH and milder reactivity.