Student Strategy Note
Acids, Bases, and Salts questions appear in every single ECZ Chemistry examination across Paper 1 (MCQ), Paper 2 (Theory), and Paper 3 (Practical titration and salt identification). Memorize the solubility rules and salt preparation recipes to guarantee full marks.
Table of Contents
1. Acid & Base Theories
In ECZ Chemistry, acids and bases are defined by two key concepts:
- Arrhenius Definition: An acid produces hydrogen ions ($H^+$ or $H_3O^+$) in aqueous solution, whereas a base produces hydroxide ions ($OH^-$).
- Brønsted-Lowry Definition: An acid is a proton donor ($H^+$ donor), and a base is a proton acceptor ($H^+$ acceptor).
2. The pH Scale & Chemical Indicators
The pH scale measures hydrogen ion concentration from 0 to 14. Strong acids (e.g., $\text{HCl}, \text{H}_2\text{SO}_4, \text{HNO}_3$) fully dissociate in water, while weak acids (e.g., ethanoic acid $\text{CH}_3\text{COOH}$) only partially ionize.
| Indicator | Color in Acid (pH < 7) | Color at Neutral (pH = 7) | Color in Alkali (pH > 7) |
|---|---|---|---|
| Litmus Paper | Red | Purple | Blue |
| Methyl Orange | Red / Pink | Orange | Yellow |
| Phenolphthalein | Colorless | Colorless | Pink / Magenta |
3. Reactions of Acids & Ionic Equations
The core chemical behaviors of dilute mineral acids include:
- Acid + Metal $\to$ Salt + Hydrogen gas: $$\text{Mg}(s) + 2\text{HCl}(aq) \to \text{MgCl}_2(aq) + \text{H}_2(g)$$
- Acid + Base/Alkali $\to$ Salt + Water (Neutralization): $$\text{H}^+(aq) + \text{OH}^-(aq) \to \text{H}_2\text{O}(l)$$
- Acid + Carbonate $\to$ Salt + Water + Carbon dioxide gas: $$2\text{H}^+(aq) + \text{CO}_3^{2-}(aq) \to \text{H}_2\text{O}(l) + \text{CO}_2(g)$$
4. Rules of Salt Solubility
| Salt Type | Solubility in Cold Water | Exceptions (Insoluble or Special) |
|---|---|---|
| Nitrates ($\text{NO}_3^-$) | ALL are soluble. | None. |
| Sodium, Potassium, Ammonium | ALL are soluble. | None. |
| Chlorides ($\text{Cl}^-$) | Most are soluble. | $\text{AgCl}$ and $\text{PbCl}_2$ (soluble in hot water). |
| Sulfates ($\text{SO}_4^{2-}$) | Most are soluble. | $\text{BaSO}_4$, $\text{PbSO}_4$, and sparingly soluble $\text{CaSO}_4$. |
| Carbonates ($\text{CO}_3^{2-}$) | Most are insoluble. | Only $\text{Na}_2\text{CO}_3$, $\text{K}_2\text{CO}_3$, $(\text{NH}_4)_2\text{CO}_3$ are soluble. |
5. Methods of Preparing Salts in the Lab
Method A: Acid + Insoluble Base/Carbonate (Excess Method)
Used to prepare soluble salts (e.g., Copper(II) Sulfate $\text{CuSO}_4$ from black copper oxide $\text{CuO}$ and dilute $\text{H}_2\text{SO}_4$). Warm the acid, add excess $\text{CuO}$ until unreacted solid remains, filter off the excess, evaporate filtrate to saturation point, and allow crystals to form.
Method B: Titration (Acid + Soluble Alkali)
Used to prepare soluble salts of sodium, potassium, or ammonium where both reactants are liquids. Use an indicator to find the exact neutralization end-point, repeat without indicator, and crystallize.
Method C: Precipitation (Insoluble Salts)
Used to prepare insoluble salts (e.g., $\text{BaSO}_4$ or $\text{PbI}_2$). Mix two soluble solutions: $$\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \to \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$ Filter off the precipitate, wash with distilled water, and dry in an oven or between filter papers.
Teacher Practical Advice
Ensure students do not evaporate salt solutions to dryness with a Bunsen burner, as this destroys water of crystallization (e.g. producing white anhydrous copper sulfate powder instead of beautiful blue hydrated crystals $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$).