Acids and Their Properties Acids are substances that produce H⁺ (hydrogen ions) in aqueous solution. Properties of acids: (1) Sour taste (e.g., lemon juice, vinegar). (2) Turn blue litmus red. (3) React with metals to produce hydrogen gas: Zn + H₂SO₄ → ZnSO₄ + H₂↑. (4) React with carbonates to produce CO₂: Na₂CO₃ + HCl → NaCl + H₂O + CO₂. (5) Conduct electricity (electrolytes). Types: Strong acids (fully ionise): HCl, H₂SO₄, HNO₃. Weak acids (partially ionise): CH₃COOH (acetic acid), H₂CO₃ (carbonic acid). Natural acids: Tartaric acid (tamarind), citric acid (lemon), lactic acid (curd), oxalic acid (tomato), formic acid (ant sting). Diluting acids: Always add acid to water (never water to acid) — "Do as you oughta, add acid to wata." The reaction is highly exothermic so adding water to concentrated acid causes violent spattering. Acid + Metal oxide → Salt + Water (neutralisation type).
Bases and Their Properties Bases are substances that produce OH⁻ (hydroxide ions) in aqueous solution. Alkalis are bases that are soluble in water. Properties of bases: (1) Bitter taste and soapy/slippery feel. (2) Turn red litmus blue. (3) React with acids to form salt and water (neutralisation). (4) React with metals like zinc and aluminium to give hydrogen: 2NaOH + Zn → Na₂ZnO₂ + H₂. (5) Conduct electricity. Common bases: NaOH (caustic soda/sodium hydroxide), KOH (potassium hydroxide), Ca(OH)₂ (lime water/slaked lime), Mg(OH)₂ (milk of magnesia — antacid), NH₄OH (ammonium hydroxide). Strong bases: NaOH, KOH (fully ionise). Weak base: NH₄OH. Natural/everyday bases: Baking soda (NaHCO₃), soap, bleaching powder Ca(ClO)Cl. Indicators: Litmus (from lichens), phenolphthalein (colourless in acid, pink in base), methyl orange (red in acid, yellow in base). Universal indicator gives a range of colours and can measure approximate pH.
pH is a measure of the concentration of H⁺ ions in a solution. pH scale runs from 0 to 14. pH < 7: Acidic solution (more H⁺ ions). pH = 7: Neutral solution (e.g., pure water). pH > 7: Basic/alkaline solution (more OH⁻ ions). Lower the pH → stronger the acid. Higher the pH → stronger the base. pH 0 is very strongly acidic, pH 14 is very strongly alkaline. Examples: HCl (conc.) pH ≈ 0; Gastric juice pH 1–2; Lemon juice pH 2–3; Coffee pH 5; Pure water pH 7; Blood pH 7.35–7.45; Baking soda pH 8–9; Soap pH 10; NaOH (conc.) pH 14. Importance of pH in everyday life: (1) Tooth decay: bacteria produce acids (pH < 5.5 destroys enamel) — toothpaste (basic) neutralises. (2) Acid rain: pH < 5.6 due to SO₂ and NO₂ — damages crops, buildings. (3) Aquatic life: needs pH 7–8.5 to survive. (4) Soil pH: crops need pH 6–7; excess acid → add lime. (5) Bee sting: formic acid (pH ~3.5) — use baking soda. Wasp sting: alkaline — use vinegar.
Neutralisation: Acid + Base → Salt + Water. This reaction is exothermic (heat is released). Example: HCl + NaOH → NaCl + H₂O. H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O. Net ionic equation: H⁺ + OH⁻ → H₂O. Salt: An ionic compound formed by the reaction of an acid and a base. The cation comes from the base, the anion from the acid. Examples: NaCl (from HCl + NaOH), CaSO₄ (from H₂SO₄ + Ca(OH)₂), NH₄Cl (from HCl + NH₄OH). Classification of salts: (1) Neutral salts: pH = 7, e.g., NaCl, KNO₃. (2) Acidic salts: pH < 7, e.g., NH₄Cl (NH₄⁺ hydrolyses to give H⁺). (3) Basic salts: pH > 7, e.g., Na₂CO₃ (CO₃²⁻ hydrolyses to give OH⁻). Family of salts: Salts of same acid: chlorides (NaCl, KCl, CaCl₂), sulphates, carbonates etc. Salts of same base: sodium salts (NaCl, Na₂SO₄, NaHCO₃).
Important Salts in Daily Life Several salts have important uses in daily life: (1) Common Salt (NaCl — Sodium Chloride): Used in cooking, preservation of food, making soap, glass. Raw material for making NaOH (by chlor-alkali process), Cl₂ and H₂. Chlor-alkali process: 2NaCl(aq) + 2H₂O → 2NaOH(aq) + Cl₂(g) + H₂(g). Products: Cl₂ for bleaching, NaOH for paper/soaps/detergents, H₂ as fuel. (2) Baking Soda (NaHCO₃ — Sodium Hydrogen Carbonate): Used in baking as a leavening agent (releases CO₂ to make bread/cake rise). Also used as antacid. Decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑. (3) Washing Soda (Na₂CO₃·10H₂O — Sodium Carbonate Decahydrate): Used in cleaning, softening hard water, manufacturing glass/soap/paper. Na₂CO₃ is obtained from NaHCO₃: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. (4) Bleaching Powder (Ca(ClO)Cl — Calcium Oxychloride): Prepared by passing Cl₂ over dry slaked lime: Ca(OH)₂ + Cl₂ → Ca(ClO)Cl + H₂O. Used to disinfect drinking water, bleach cotton and linen, and as oxidising agent. (5) Plaster of Paris (CaSO₄·½H₂O): Prepared by heating gypsum: CaSO₄·2H₂O → CaSO₄·½H₂O + 3/2 H₂O. Used in making casts for fractured bones, chalk, fire-proofing materials.
Indicators show whether a substance is an acid or a base by a change in colour. Litmus: blue litmus turns red in acid; red litmus turns blue in base. Phenolphthalein: colourless in acid, pink in base. Methyl orange: red in acid, yellow in base. Turmeric turns red in base. Olfactory indicators change smell: onion, vanilla and clove oil lose their smell in a base. Universal indicator shows a range of colours that correspond to the pH value.
(1) Acid + metal → salt + hydrogen: Zn + H₂SO₄ → ZnSO₄ + H₂ (hydrogen burns with a pop sound). (2) Acid + metal carbonate / hydrogencarbonate → salt + CO₂ + water: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂; NaHCO₃ + HCl → NaCl + H₂O + CO₂. The CO₂ turns lime water milky. (3) Acid + base → salt + water (neutralisation): NaOH + HCl → NaCl + H₂O. (4) Acid + metal oxide → salt + water: CuO + 2HCl → CuCl₂ + H₂O (metal oxides are basic).
(1) Base + acid → salt + water. (2) Base + non-metal oxide → salt + water: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O, showing that non-metal oxides are acidic. (3) Some metals react with alkalis to give hydrogen: 2NaOH + Zn → Na₂ZnO₂ + H₂. A base that dissolves in water is called an alkali (NaOH, KOH, Ca(OH)₂). All alkalis are bases, but not all bases are alkalis. Bases feel soapy and taste bitter.
Acids produce hydrogen ions H⁺(aq), present as hydronium ions H₃O⁺, in water; bases produce hydroxide ions OH⁻. Dry HCl gas does not change the colour of dry litmus paper because it does not release H⁺ ions without water. Glucose and alcohol contain hydrogen but are not acids, since they do not give H⁺ in water. Dilution: mixing an acid or a base with water decreases the concentration of H₃O⁺ or OH⁻ per unit volume. It is highly exothermic, so always add acid to water slowly with constant stirring, never water to acid.
The pH scale (0 to 14) measures the hydrogen ion concentration; p stands for "potenz" (power) in German. pH below 7: acidic; pH 7: neutral; above 7: basic. Lower pH means a stronger acid; higher pH means a stronger base. Approximate values: gastric juice 1.2, lemon juice 2.2, vinegar about 3, tomato 4.2, black coffee 5, milk 6.4, pure water 7, blood 7.4, baking soda solution 8.2, milk of magnesia 10, sodium hydroxide 14. Universal indicator or pH paper gives an approximate pH.
Common Salt and Chlor-Alkali Process Sodium chloride (common salt) is obtained from sea water and rock salt. When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g). This is the chlor-alkali process. Products: chlorine at the anode, hydrogen at the cathode and sodium hydroxide near the cathode. Uses: H₂ for fuels and margarine; Cl₂ for water treatment and bleach; NaOH for soaps and detergents; bleaching powder.
Bleaching powder (calcium oxychloride, CaOCl₂) is produced by the action of chlorine on dry slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Uses: (1) bleaching cotton and linen in the textile industry, wood pulp in paper factories; (2) bleaching washed clothes in laundry; (3) oxidising agent in many chemical industries; (4) disinfecting drinking water to make it free of germs.
Baking Soda and Washing Soda Baking soda (sodium hydrogencarbonate, NaHCO₃) is made as NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃. On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Uses: baking powder (with tartaric acid, makes cakes soft and spongy), antacid, soda-acid fire extinguishers. Washing soda (Na₂CO₃·10H₂O) is obtained by recrystallising sodium carbonate: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. Uses: glass, soap and paper industries, manufacture of borax, removing permanent hardness of water, and cleaning.
Plaster of Paris and Water of Crystallisation Plaster of Paris (CaSO₄·½H₂O) is made by heating gypsum (CaSO₄·2H₂O) at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. When mixed with water it sets into a hard solid gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Uses: plasters for fractured bones, toys, decorative materials, smoothing surfaces. It must be stored in a moisture-proof container. Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt: CuSO₄·5H₂O (blue, turns white on heating), Na₂CO₃·10H₂O, CaSO₄·2H₂O.
Summary: Acids, Bases and Salts Acids: sour, turn blue litmus red, release H⁺, pH below 7. Bases: bitter, soapy, turn red litmus blue, release OH⁻, pH above 7. Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (CH₃COOH) partly. Salts of strong acid and strong base are neutral (pH 7); strong acid and weak base are acidic (pH below 7); strong base and weak acid are basic (pH above 7). Salt families: NaCl, NaNO₃ (sodium family); NaCl, KCl (chloride family). Tooth decay begins when mouth pH falls below 5.5. Key salts: NaCl, NaOH, CaOCl₂, NaHCO₃, Na₂CO₃·10H₂O, CaSO₄·½H₂O.
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