Acids are the substances which taste sour, change blue litmus red, are corrosive to metals and furnish H+ ions in their aqueous solutions. Bases are the substances which taste bitter, change red litmus blue, feel slippery and furnish OH ions in their aqueous solutions.

HCl (aq) → H+ (aq) + Cl (aq)

NaOH (aq) → Na+ (aq) + OH (aq)

Indicators are the substances that show one colour in an acidic medium and another colour in a basic medium. Litmus, phenolphthalein and methyl orange are commonly used indicators.

Aqueous solutions of acids and bases both conduct electricity as they dissociate on dissolving in water and liberate cations and anions which help in conducting electricity. Strong acids and bases dissociate completely in water. Weak acids and bases dissociate partially in water.

Acids and bases react with each other to produce salt and water. Such reactions are called neutralization reactions.

HCl (aq) + NaOH (aq) → NaCl + H2O (l)

Water itself undergoes dissociation and furnishes H+ and OH ions in equal numbers. This is called self dissociation of water. Concentrations of H+ and OH ion formed by
the self dissociation of water are 1.0 × 10–7 molar each at 25°C.

In pure water or in any aqueous solution, pH + pOH = pKw = 14 at 25°C. In pure water [H+] = [OH]. It is also true in any neutral aqueous solution. In terms of pH, pH = pOH = 7 in water and any neutral solution.

In acidic solution [H+] > [OH] and pH < pOH. Also pH < 7 at 25°C. In basic solutions [H+] < [OH] and pH > pOH. Also pH > 7 at 25°C.

pH Scale: The pH scale ranges from 0 to 14. The pH is the logarithm of the reciprocal of the hydrogen ion concentration.

pH = – log [H+]

Because of the negative sign in the expression, if [H+] increases, pH would decrease and if it decreases, pH would increase.

Important Compounds

  • Baking Soda (NaHCO3)
  • Washing soda (Na2CO3.10H2O)
  • Plaster of Paris 2CaSO4.H2O or CaSO4.½H2O
  • Bleaching Powder (CaOCl2)