Acids & Bases
Acids and bases are fundamental to chemistry. Three theories define them, the pH scale measures their strength, and neutralisation combines them into salts and water.
A. Definitions — Three Theories
Arrhenius, Brønsted-Lowry & Lewis
| Theory | Acid | Base | Limitation |
|---|---|---|---|
| Arrhenius | Produces H⁺ ions in aqueous solution | Produces OH⁻ ions in aqueous solution | Only applies to aqueous (water) solutions |
| Brønsted-Lowry | Proton (H⁺) donor — gives away H⁺ | Proton (H⁺) acceptor — receives H⁺ | More general — works in non-aqueous systems |
| Lewis | Electron pair acceptor | Electron pair donor | Most general theory — includes reactions without H⁺ transfer |
Arrhenius
AcidProduces H⁺ ions in aqueous solution
BaseProduces OH⁻ ions in aqueous solution
LimitationOnly applies to aqueous (water) solutions
Brønsted-Lowry
AcidProton (H⁺) donor — gives away H⁺
BaseProton (H⁺) acceptor — receives H⁺
LimitationMore general — works in non-aqueous systems
Lewis
AcidElectron pair acceptor
BaseElectron pair donor
LimitationMost general theory — includes reactions without H⁺ transfer
⚡ MCQ Tip Arrhenius: H⁺/OH⁻ in water. Brønsted-Lowry: proton donor/acceptor (most exam-relevant). Lewis: electron pair acceptor/donor (most general). A substance can be amphiprotic — acts as both acid and base (e.g. H₂O, HCO₃⁻).
B. Properties of Acids & Bases
Comparison Table
| Property | Acids | Bases |
|---|---|---|
| Taste | Sour (e.g. lemon = citric acid) | Bitter (e.g. baking soda) |
| pH | Less than 7 | Greater than 7 |
| Blue litmus paper | Turns RED | No change (stays blue) |
| Red litmus paper | No change | Turns BLUE |
| Methyl orange indicator | Turns red/pink | Turns yellow/orange |
| Phenolphthalein indicator | Colourless | Turns pink/magenta |
| Reaction with metals | Acid + metal → salt + H₂ gas ↑ | Bases generally don't react with metals |
| Texture/feel | Sharp, corrosive | Slippery, soapy feel |
C. The pH Scale
pH — Measure of Acidity/Alkalinity
pH Formula
pH = −log[H⁺]
Each unit change in pH = 10× change in H⁺ concentration. pH 3 is 10× more acidic than pH 4.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
← ACIDIC (0–6)
NEUTRAL (7)
ALKALINE (8–14) →
| Substance | Approx. pH |
|---|---|
| Stomach acid (HCl) | ~1–2 |
| Lemon juice (citric acid) | ~2–3 |
| Vinegar (acetic acid) | ~3 |
| Coffee | ~5 |
| Pure water | 7 (neutral) |
| Blood | ~7.4 |
| Baking soda (NaHCO₃) | ~8–9 |
| Ammonia solution | ~11 |
| NaOH solution | ~13–14 |
D. Strong vs Weak Acids & Bases
Ionisation & Examples
| Type | Ionisation | Examples |
|---|---|---|
| Strong Acids | Fully ionise (100%) in water → maximum H⁺ ions | HCl (hydrochloric), H₂SO₄ (sulfuric), HNO₃ (nitric) |
| Weak Acids | Partially ionise (small %) → equilibrium between ionised and non-ionised | CH₃COOH (acetic/vinegar), H₂CO₃ (carbonic), citric acid |
| Strong Bases | Fully dissociate in water → maximum OH⁻ ions | NaOH (caustic soda), KOH (potassium hydroxide) |
| Weak Bases | Partially ionise in water | NH₃ (ammonia), Mg(OH)₂ |
⚡ MCQ Tip Strong ≠ concentrated. Strong = degree of ionisation (100%). Concentrated = amount of solute. A dilute strong acid and a concentrated weak acid can have the same pH!
E. Neutralisation & Titration
Neutralisation Reactions
General Neutralisation
Acid + Base → Salt + Water
Example
HCl + NaOH → NaCl + H₂O
Hydrochloric acid + Sodium hydroxide → Sodium chloride (table salt) + Water
- Titration: laboratory technique to find the unknown concentration of an acid or base
- A solution of known concentration (standard solution) is added from a burette to the unknown solution
- An indicator changes colour at the equivalence point (end point) when moles of acid = moles of base
- Phenolphthalein: colourless in acid, pink in base — used for strong base vs weak acid titrations
- Methyl orange: red in acid, yellow in base — used for strong acid vs weak base titrations
Quick MCQ Revision
| Fact | Answer |
|---|---|
| pH formula | pH = −log[H⁺] |
| pH < 7 | Acidic |
| pH = 7 | Neutral (pure water at 25°C) |
| pH > 7 | Basic/Alkaline |
| Blue litmus in acid | Turns RED |
| Red litmus in base | Turns BLUE |
| Strong acids (3) | HCl, H₂SO₄, HNO₃ — fully ionise |
| Weak acid example | CH₃COOH (acetic acid/vinegar) — partially ionises |
| Neutralisation | Acid + Base → Salt + Water |
| Brønsted-Lowry acid | Proton (H⁺) donor |