CBSE- Acids, Bases and Salts-Notes

Their definitions in term of furnishing of H+ and OH- ions, General properties, examples and uses, Concept of pH scale (Definition relating to logarithm not required), Importance of pH in everyday life; Preparation and uses of Sodium hydroxide, Bleaching powder, Washing soda and Plaster of Paris.

Acids : Acids are sour in taste, turn blue litmus to red, dissolve in water to release H⁺ ions.

E.g., vinegar, hydrochloric acid and sulphuric acid.

            •   Reaction with Metal

Acid + Metal → Salt + Hydrogen gas

e.g., 2HCl + Zn → ZnCl₂ + H₂

            •   Reaction with Metal carbonate

Acid + Metal carbonate → Salt + CO₂ + H₂O

e.g., 2HCl + Na₂CO₃ → 2NaCl + CO₂ + H₂O

            •   Reaction with Metal hydrogen carbonate

Acid + Metal hydrogen carbonate Salt + CO₂ + H₂O

e.g. HCl + NaHCO₃ → NaCl + H₂O + CO₂

            •   Reaction with Metallic oxide

Acid + Metal oxide → Salt + Water

e.g. 2HCl + CuO → CuCl₂ + H₂O

            •   Acids in water:

Acids produce H⁺ ions when dissolved in water. H+ ions cannot exist alone. They combine with water molecule (H₂O) to form H₃O⁺ (hydronium ions). It conducts electricity.

            •   Decrease in H₃0+ ions concentration per unit volume results in formation of dilute acids.

            •   It is a highly exothermic reaction.

Acids when dissolved in water release large amount of heat. If water is added to concentrated acid then the heat generated may cause the mixture to splash out and cause bums. Hence to avoid burns acid must be added drop wise into water with constant stirring. So that the heat generated spreads over in water.

strong acids → release more H⁺ ions → HCl

weak acids → releases less number of H⁺ ions → acetic acid

strong base → give more OH– ions → NaOH

weak base → gives less OH– ions → CH₃COOH

Bases: Bases are bitter in taste, turns red litmus to blue and when dissolved in water releases OH– ions;

e.g., NaOH and KOH.

            •   Reaction with metal

            Base + Metal → Salt + H₂ gas

e.g., 2NaOH + Zn → Na₂ZnO₂ + H₂

This reaction is possible only with reactive metals like sodium and potassium.

            •   Reaction with non-metallic oxide

            Base + Non-metallic oxide → Salt + H₂O

Bases in water → Bases produce OH– ions when dissolved in water. Bases soluble in water are called alkalis. It conducts electricity.

            •   Decrease in OH– ions single concentration per unit volume results in formation of dilute bases.

            •   It is a exothermic reaction.

To make basic solution, base must be added drop wise into water with constant stirring, so that the heat generated spreads over in water.

Indicators: Indicators are those substances which tell us whether a substance is acidic or basic by change in colour. For e.g., litmus solution.

            •   Olfactory indicators: Those substances whose odour changes in acidic or basic media are called lfactory indicators. For e.g., clove, vanilla, onion.

            •   Natural indicators: Turmeric, litmus (obtained from lichen)

            •   Synthetic indicators: Methyl orange, phenolphthalein.