Acids, Bases and Salts Class 10 Notes
Revise indicators, pH, neutralisation and everyday salts with a compact Class 10 guide, worked equations and practice answers.
Acids, bases and salts are easier to organise by what their aqueous solutions do. Acids provide hydrogen ions in water; these associate with water as hydronium ions. Bases such as sodium hydroxide provide hydroxide ions. Indicators help identify acidic or basic behaviour without tasting a substance.
Indicators and pH
| Check | Acidic solution | Basic solution |
|---|---|---|
| Litmus | Blue turns red | Red turns blue |
| pH at ordinary classroom conditions | Below 7 | Above 7 |
| Neutral example | Pure water is approximately pH 7 at 25°C | Neutral means neither acidic nor basic |
pH is logarithmic: a one-unit decrease corresponds to ten times the hydrogen-ion concentration in the usual dilute-solution interpretation. Do not confuse acid strength with concentration. Strength concerns ionisation; concentration concerns how much solute is present in a given amount of solution.
Three useful reaction patterns
Neutralisation
- 1HCl + NaOH → NaCl + H₂O.
- 2An acid and a base react to form a salt and water. This is a useful pattern for many school-level neutralisation examples.
Acid and carbonate
- 1Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
- 2The carbonate produces a salt, water and carbon dioxide. Check Na: 2, C: 1, O: 3, H: 2, Cl: 2 on each side.
Acid and a suitable metal
- 1Zn + 2HCl → ZnCl₂ + H₂.
- 2Zinc and dilute hydrochloric acid form a salt and hydrogen. This pattern does not mean every metal reacts with every acid in the same way.
Salts to distinguish
| Name | Formula | One use or feature |
|---|---|---|
| Baking soda | NaHCO₃ | Sodium hydrogen carbonate; releases CO₂ when heated or reacted with acid. |
| Washing soda | Na₂CO₃·10H₂O | Hydrated sodium carbonate; used in cleaning and softening hard water. |
| Gypsum | CaSO₄·2H₂O | Hydrated calcium sulphate. |
| Plaster of Paris | CaSO₄·½H₂O | Forms gypsum when mixed with water and sets. |
Practice and answers
- 1. Which is more acidic, pH 3 or pH 5? Answer: pH 3; its hydrogen-ion concentration is about 100 times that at pH 5.
- 2. Complete H₂SO₄ + 2NaOH → … Answer: Na₂SO₄ + 2H₂O.
- 3. Why does the formula for washing soda include 10H₂O? Answer: it represents water of crystallisation in the crystal structure.
- 4. Does a neutral solution have no ions? Answer: no. Neutrality concerns the balance of hydrogen and hydroxide ion concentrations, not the absence of ions.
Read chemical labels and follow school lab instructions. Never identify an unknown acid or base by taste or touch. When revising, use written equations and indicator tables; handling reagents is not required to complete these questions.