Acids, Bases and Salts

Chapter at a glance

• Acids are sour, turn blue litmus red and produce H+(aq)/H3O+ ions in aqueous solution; bases are bitter, turn red litmus blue and produce OH–(aq) ions.
• All acids generate H+(aq) ions and all bases generate OH–(aq) ions in water; only aqueous solutions conduct electricity.
• Acids react with metals to give salt + H2(g); metal carbonates/hydrogencarbonates give salt + CO2 + H2O.
• Neutralisation: acid + base → salt + water; metal oxides behave as basic oxides.
• pH scale (0–14) measures H+ concentration; pH < 7 acidic, pH = 7 neutral, pH > 7 basic; living organisms survive only in narrow pH range.
• Mixing concentrated acid/base with water is highly exothermic; acid is always added to water.
• Salts of strong acid + strong base are neutral (pH 7); strong acid + weak base acidic (pH < 7); strong base + weak acid basic (pH > 7).
• Water of crystallisation is fixed number of water molecules in one formula unit of salt; examples include CuSO4·5H2O and CaSO4·2H2O (gypsum).

Definitions and laws

• “The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. In general, a neutralisation reaction can be written as – Base + Acid → Salt + Water.”
• “Bases which are soluble in water are called alkalis.”
• “Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.”
• “Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids.”
• “On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline)… The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. As the pH value increases from 7 to 14, it represents an increase in OH– ion concentration…”
• “Acid + Metal → Salt + Hydrogen gas”
• “Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water”
• “Metal oxide + Acid → Salt + Water”
• “HCl + H2O → H3O+ + Cl–”; “H+ + H2O → H3O+”
• “NaOH(s) → Na+(aq) + OH–(aq)” (similar for KOH, Mg(OH)2)

Important diagrams and activities

• Fig. 2.1: Reaction of zinc granules with dilute H2SO4 and testing H2 by burning – demonstrates acid + metal → salt + H2.
• Fig. 2.2: Passing CO2 through Ca(OH)2 (lime water) – shows metal carbonate/hydrogencarbonate + acid → CO2 (turns lime water milky).
• Fig. 2.3: Acid solution in water conducts electricity (bulb glows) – shows ions carry current.
• Fig. 2.4: Preparation of HCl gas and testing with dry/wet litmus – shows H+ ions form only in presence of water.
• Fig. 2.6 & 2.7: pH scale and pH of common substances – illustrates variation of pH with H+/OH– concentration.
• Activity 2.3/2.4: Zinc with dilute acid/NaOH – produces H2 gas (tested by burning).
• Activity 2.5: Na2CO3/NaHCO3 + dilute HCl → CO2 (lime water test).
• Activity 2.6: NaOH + phenolphthalein + HCl (colour change and reversal) – demonstrates neutralisation.
• Activity 2.8: Conductivity of acid vs glucose/alcohol solutions – only acids ionise to give H+.
• Activity 2.10: Dilution of conc. H2SO4/NaOH – highly exothermic process.
• Activity 2.15: Heating CuSO4·5H2O crystals – shows loss and regain of water of crystallisation.
• Fig. 2.8 & chlor-alkali process: Electrolysis of brine → NaOH, Cl2, H2 and their uses.

Common misconceptions and exam pitfalls

• All compounds containing hydrogen are acids (only those producing H+(aq) are acids; glucose/alcohol do not).
• Dry HCl gas is acidic (it shows no effect on dry litmus; water is required for H+ formation).
• Water should be added to concentrated acid (always add acid to water with stirring to avoid splashing/excessive heat).
• pH 7 is “not acidic” – it is exactly neutral; living organisms tolerate only 7.0–7.8.
• All salts are neutral (pH depends on parent acid/base strength).
• Formula of Plaster of Paris written incorrectly (CaSO4·½H2O, not 2H2O).

Formula sheet