pH scales, indicators, neutralisation, titration, and salt preparation — the chemistry of everyday life! 🌡️🧪
| Theory | Acid | Base |
|---|---|---|
| Arrhenius | Produces H⁺ ions in water HCl → H⁺ + Cl⁻ | Produces OH⁻ ions in water NaOH → Na⁺ + OH⁻ |
| Brønsted-Lowry | A proton (H⁺) donor | A proton (H⁺) acceptor |
| Strength | Meaning | Acid Examples | Base Examples |
|---|---|---|---|
| Strong | Fully/completely ionises in water | HCl, H₂SO₄, HNO₃ | NaOH, KOH, Ca(OH)₂ |
| Weak | Partially ionises (equilibrium) | CH₃COOH (ethanoic), H₂CO₃ (carbonic), citric acid | NH₃ (ammonia solution) |
The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14:
The pH scale is logarithmic — each unit change represents a 10× change in H⁺ concentration. pH 1 is 100× more acidic than pH 3.
| Indicator | Acidic (Low pH) | Neutral (~7) | Alkaline (High pH) | Best for |
|---|---|---|---|---|
| Litmus | Red | Purple | Blue | General acid/alkali test |
| Methyl orange | Red | Orange | Yellow | Strong acid vs strong/weak base titrations |
| Phenolphthalein | Colourless | Colourless | Pink/magenta | Weak acid vs strong base titrations |
| Universal indicator | Red → orange → yellow | Green | Blue → violet | Estimating pH value |
| Reaction Type | General Equation | Example |
|---|---|---|
| Acid + Metal | Acid + Metal → Salt + H₂↑ | Zn + H₂SO₄ → ZnSO₄ + H₂↑ |
| Acid + Metal Oxide (base) | Acid + Metal Oxide → Salt + H₂O | CuO + H₂SO₄ → CuSO₄ + H₂O |
| Acid + Metal Hydroxide (base) | Acid + Metal Hydroxide → Salt + H₂O | NaOH + HCl → NaCl + H₂O |
| Acid + Metal Carbonate | Acid + Metal Carbonate → Salt + H₂O + CO₂↑ | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ |
| Acid + Metal Hydrogen carbonate | Acid + MHCO₃ → Salt + H₂O + CO₂↑ | NaHCO₃ + HCl → NaCl + H₂O + CO₂↑ |
| Acid + Ammonia | Acid + NH₃ → Ammonium salt | HCl + NH₃ → NH₄Cl |
A salt is formed when the H⁺ of an acid is replaced by a metal ion (or NH₄⁺).
| Acid Used | Salt Name Ending | Example |
|---|---|---|
| Hydrochloric acid (HCl) | …chloride | NaCl (sodium chloride) |
| Sulfuric acid (H₂SO₄) | …sulfate | CuSO₄ (copper sulfate) |
| Nitric acid (HNO₃) | …nitrate | Ca(NO₃)₂ (calcium nitrate) |
| Phosphoric acid (H₃PO₄) | …phosphate | Na₃PO₄ (sodium phosphate) |
| Carbonic acid (H₂CO₃) | …carbonate | Na₂CO₃ (sodium carbonate) |
| Method | Used For | Steps |
|---|---|---|
| Titration then evaporation | Soluble salts from soluble acid + soluble base (e.g. NaCl from HCl + NaOH) | 1. Titrate to find exact volumes. 2. Mix the same volumes WITHOUT indicator. 3. Evaporate/crystallise to get pure salt. |
| Excess solid + filtration | Soluble salts from insoluble base/carbonate + acid | 1. Add excess solid (CuO/CaCO₃) to acid until no more dissolves. 2. Filter off excess solid. 3. Evaporate/crystallise salt solution. |
| Direct combination | Some salts made directly from elements | E.g. 2Na + Cl₂ → 2NaCl |
| Precipitation | Insoluble salts (e.g. BaSO₄, PbI₂) | Mix two solutions — insoluble salt precipitates. Filter, wash, dry. |
Titration is used to find the exact volume of one solution that reacts completely with a known volume of another — then calculate the unknown concentration.
c₁V₁ / n₁ = c₂V₂ / n₂| Ion | Soluble? | Exceptions |
|---|---|---|
| All nitrates (NO₃⁻) | ✅ All soluble | None |
| All sodium/potassium/ammonium salts | ✅ All soluble | None |
| Chlorides (Cl⁻) | ✅ Most soluble | AgCl, PbCl₂ (insoluble) |
| Sulfates (SO₄²⁻) | ✅ Most soluble | BaSO₄, PbSO₄, CaSO₄ (insoluble) |
| Carbonates (CO₃²⁻) | ❌ Most insoluble | Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ (soluble) |
| Hydroxides (OH⁻) | ❌ Most insoluble | NaOH, KOH, Ba(OH)₂ (soluble) |
Drag the slider to change the pH (0–14). Watch the universal indicator colour, all indicator panels, and common examples update live!
Select the type of salt you want to make — get the correct method and steps instantly!
👆 Select a salt type above
HCl (acid) added to NaOH (base). Watch the S-curve draw itself and see indicator ranges.
Click to flip!
Problem: 25.0 cm³ of H₂SO₄ is titrated against 0.200 mol/dm³ NaOH. The titre is 20.0 cm³ of NaOH. The reaction is: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Find the concentration of the H₂SO₄.
V(NaOH) = 20.0 cm³ = 0.020 dm³. c(NaOH) = 0.200 mol/dm³. Use n = c × V.
From the equation H₂SO₄ : NaOH = 1:2. So moles of H₂SO₄ = moles of NaOH ÷ 2.
V(H₂SO₄) = 25.0 cm³. Convert to dm³ by dividing by 1000.
Use c = n / V. n = 0.002 mol, V = 0.025 dm³.
Click a term, then its match. Green = correct!
1 (a) A Brønsted-Lowry acid is a proton (H⁺) donor. ✓
2 (b) HNO₃ is the acid (it donates H⁺ to water) ✓; H₂O is the base (it accepts H⁺ to become H₃O⁺) ✓
3 (c) A base is any substance that accepts protons/reacts with acids. ✓ An alkali is a base that dissolves in water to produce OH⁻ ions. ✓ All alkalis are bases but not all bases are alkalis (e.g. CuO is a base but is insoluble so not an alkali). ✓
1 (a) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) ✓
2 (b) n(NaOH) = 0.100 × (22.5/1000) = 0.00225 mol ✓ | Mole ratio 1:1 → n(HCl) = 0.00225 mol ✓
3 c(HCl) = 0.00225 / (25.0/1000) = 0.00225 / 0.025 = 0.09 mol/dm³ ✓
4 (c) Either methyl orange or phenolphthalein ✓ — both give a sharp colour change because this is a strong acid + strong base titration. ✓
1 Add excess CuO (black powder) to warm dilute H₂SO₄ in a beaker. ✓
2 Stir until no more CuO dissolves (excess solid present = all acid has reacted). ✓
3 Filter the mixture to remove excess CuO (residue). ✓
4 Heat the filtrate (blue CuSO₄ solution) gently until saturated — test by dipping a glass rod, small crystals form. ✓
5 Allow to cool slowly; crystals form. Filter off crystals, pat dry with filter paper, leave to dry. ✓
1 (a) HCl → colourless (pH ~1, acidic) ✓
2 (b) Distilled water → colourless (pH 7, neutral) ✓
3 (c) NaOH → pink/magenta (pH ~13, alkaline) ✓
4 At the equivalence point of weak acid + strong base, the pH is ABOVE 7 (because the salt formed is basic). ✓ Phenolphthalein changes colour in this alkaline region (pH 8.2–10). Methyl orange changes at pH 3.1–4.4 (too acidic) and would change before the equivalence point. ✓
1 (a) Full: HCl + NaOH → NaCl + H₂O | Net ionic: H⁺(aq) + OH⁻(aq) → H₂O(l) ✓✓
2 (b) Full: H₂SO₄ + CuO → CuSO₄ + H₂O | Net ionic: 2H⁺(aq) + CuO(s) → Cu²⁺(aq) + H₂O(l) ✓✓
3 (c) Full: 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂ | Net ionic: 2H⁺(aq) + CaCO₃(s) → Ca²⁺(aq) + H₂O(l) + CO₂(g) ✓✓
H⁺(aq) + OH⁻(aq) → H₂O(l)
Universal net ionic equation
c₁V₁/n₁ = c₂V₂/n₂
1:1 → c₁V₁ = c₂V₂
0–6 acid | 7 neutral | 8–14 alkali
MO: red→orange→yellow
PP: colourless→pink (pH>8)
HCl → …chloride
H₂SO₄ → …sulfate
HNO₃ → …nitrate
Strong acids: HCl, H₂SO₄, HNO₃
Weak acid: CH₃COOH, H₂CO₃
Strong bases: NaOH, KOH
All nitrates ✅
Chlorides ✅ (except AgCl, PbCl₂)
Carbonates ❌ (except Na, K, NH₄)