15. Acids and Bases

Definitions
  • An acid is a compound that produces hydrogen ions (H+) when dissolved in water. H+ ions are responsible for acidic properties.
  • A base is a metallic oxide or metallic hydroxide.
  • An alkali is a soluble base that produces hydroxide ions (OH-) when dissolved in water. 
  • The pH is a measure of how acidic or alkaline a solution is in water. The pH scale is numbered from 0 (most acidic) to 14 (most alkaline).
  • A universal indicator is a mixture that gives different colours at different pH.

pH Scale
  • pH 7 is neutral. [Concentration of H+ ions = Concentration of OH- ions.]
  • pH less than 7 is acidic. [Concentration of H+ ions > Concentration of OH- ions.]
  • pH more than 7 is alkaline. [Concentration of H+ ions < Concentration of OH- ions.]

Difference between Strong and Weak Acids
  • In a strong acid, all the acid molecules are completely ionized when dissolved in water. e.g. HCl
  • In a weak acid, the acid molecules are only partially ionized when dissolved in water. e.g. CH3COOH(aq)
  • A strong acid contains a higher concentration of H+ ions than a weak acid of the same concentration.

Reactions of Acids
  • Acid + metals:    Acids react with metals (Mg, Zn and Fe) to form salt and hydrogen.
  • Acid + carbonate or hydrogencarbonates: Acids react with carbonates or hydrogencarbonates to produce a salt, carbon dioxide and water.
  • Acid + base/ alkali (neutralisation reaction): Acids neutralise base/alkalis to form salt and water only.

Reactions of Bases
  • Base + ammonium salt:    Bases react with ammonium salts to form a salt, water and ammonia gas.

Effect of pH on soil
  • To reduce acidity, calcium oxide (quicklime), calcium hydroxide (slake lime) or calcium carbonate can be added.
  • Soluble bases like sodium hydroxide should not be added to neutralise acidic soil as there is a possibility of adding excess of it which will result in soil becoming too alkaline.

Nature of Oxides
Oxides can be classified as acidic, neutral, basic or amphoteric based on metallic/non-metallic character.
  • Acidic: Non-metallic oxides - CO2  ,SO2 , NO2, P4O10
  • Neutral: Non-metallic oxides - CO , NO, H2O   
  • Basic: Metallic oxides - Na2O, MgO, CaO, FeO
  • Amphoteric: Metallic oxides - Al2O3, PbO, ZnO (exhibit both acidic and basic properties)