QA

• Test for cations involves adding aqueous sodium hydroxide and/or aqueous ammonia to identify specific cations through the formation of precipitates and their subsequent solubility
• All cations (except Na+, K+, and NH4+) give precipitates with aqueous sodium hydroxide and aqueous ammonia
• The test for cations involves two steps: adding the test reagent dropwise and then adding it in excess
• Effects of adding test reagents for cations:

  • Ammonium: No precipitate forms, on heating, ammonia gas evolves and turns red litmus paper blue
  • Calcium: White precipitate forms, insoluble in excess
  • Copper(II): Light blue precipitate forms, insoluble in excess, dissolves in excess to form a dark blue solution
  • Iron(II): Green precipitate forms, insoluble in excess, turns brown on standing
  • Iron(III): Red-brown precipitate forms, insoluble in excess
  • Zinc: White precipitate forms, dissolves in excess to form a colorless solution
• • Aluminium: White precipitate forms, dissolves in excess to form a colorless solution
  • Lead(II): White precipitate forms, dissolves in excess to form a colorless solution
• Adding excess test reagent is done because some precipitates react with the excess reagents to form soluble compounds
• An anion can be identified by observing the formation and color of the precipitate, if any, and effervescence of a gas
• Effects of adding test reagents for anions:

  • Carbonate: Effervescence of a colorless, odourless gas which forms a white precipitate when bubbled into limewater
  • Chloride: White precipitate (AgCl) forms
  • Iodide: Yellow precipitate (AgI) forms
  • Sulfate: White precipitate (BaSO4) forms
  • Nitrate: Effervescence of a colorless, pungent gas which turns damp red litmus paper blue
• Nitric acid is added to remove any carbonate and hydroxide ions present to avoid false positives
• Hydrogen: Gas extinguishes burning splint with a 'pop' sound

  <|>Carbon dioxide: White precipitate is formed, dissolves upon further bubbling

  <|>Oxygen: Glowing splint relights

  <|>Ammonia: Damp red litmus paper turns blue

  <|>Sulfur dioxide: Purple acidified KMnO4 turns colorless

  <|>Chlorine: Damp blue litmus paper turns red and is then bleached