chemistry

• Experiment 14.3


  Investigating the action of dilute alkalis on ammonium compounds
• Dilute sodium hydroxide solution reacts with ammonium chloride to form ammonia, which turns moist red litmus paper blue
• Experiment 14.4


  Investigating the action of dilute alkalis on metal ions in aqueous solution
• Most metal hydroxides are insoluble or slightly soluble in water
• When aqueous solutions of alkalis are added to solutions containing certain metal ions, metal hydroxides would be precipitated out
• Ionic equation


  Metal ion(aq) + 2OH(aq) → Metal hydroxide(s)
• Excess aqueous ammonia can dissolve some metal hydroxides and form complex ions
• Metal hydroxides that dissolve in excess aqueous ammonia


  • Zinc hydroxide
  • Copper(II) hydroxide
• Zinc hydroxide is a white precipitate that dissolves in excess aqueous ammonia to give a colourless solution
• Copper(II) hydroxide is a pale blue precipitate that dissolves in excess aqueous ammonia to give a deep blue solution
• The colour and the solubility of a precipitate in excess sodium hydroxide solution and aqueous ammonia may help identify the metal ions present
• Distinguishing two metal compounds by chemical tests

  1. Dissolve the solids in water

  2. Add sodium hydroxide solution until in excess

  3. Observe the precipitates and their solubility in excess sodium hydroxide
• A white precipitate of Pb(OH)2 forms initially, but only the Pb(OH)2 precipitate dissolves in excess sodium hydroxide solution
• Aqueous ammonia contains hydroxide ions (OH-)
• When a small amount of sodium hydroxide or aqueous ammonia is added to a solution, a precipitate may form
• Precipitates formed when a small amount of NaOH(aq) or NH3(aq) is added

  • Insoluble: Ca(OH)2, Mg(OH)2, Al(OH)3, Fe(OH)3
  • Soluble in excess NaOH(aq): Al(OH)3, Zn(OH)2, Pb(OH)2
  • Soluble in excess NH3(aq): Zn(OH)2, Cu(OH)2
• Aluminium hydroxide, zinc hydroxide and lead hydroxide are white precipitates that dissolve in excess sodium hydroxide solution
• The dissolved precipitates form complex ions in the excess sodium hydroxide solution
• Experiment


  A procedure or set of steps performed to observe or demonstrate a phenomenon
• Dilute alkalis


  Alkalis that have been mixed with a large amount of water, reducing their concentration
• Action


  The chemical reaction that occurs when dilute alkalis are added to ammonium compounds
• Ammonium compounds


  Compounds that contain the ammonium ion (NH4+)
• Ammonia gas (NH3)


  The gas produced when the alkali reacts with the ammonium compound
• pH


  A measure of the acidity or basicity of a solution
• Action


  The overall chemical reaction between dilute alkalis and ammonium compounds, which produces ammonia gas and water.
• Precipitate


  A solid that is formed when two solutions are mixed together and a chemical reaction occurs. In the case of the reaction between dilute alkalis and ammonium compounds, there is no precipitate formed.
• Action


  Refers to the overall chemical reaction between dilute alkalis and ammonium compounds, which produces ammonia gas and water.
• Precipitate


  Refers to a solid that is formed when two solutions are mixed together and a chemical reaction occurs. In the case of the reaction between dilute alkalis and ammonium compounds, there is no precipitate formed.
• Action


  The process of adding dilute alkalis to ammonium compounds, resulting in the production of ammonia gas and water.
• Precipitate


  A solid that is formed during a chemical reaction between two solutions. In the case of the reaction between dilute alkalis and ammonium compounds, there is no precipitate formed.
• Precipitate


  A solid that is formed when two solutions are mixed together and a chemical reaction occurs. The solid is formed when the reactants combine in such a way that the resulting product is no longer soluble in the solution.
• Precipitate


  A solid that forms during a chemical reaction between two solutions. The solid is formed when the reactants combine in such a way that the resulting product is no longer soluble in the solution.
• Precipitate


  A solid that is produced when two solutions are mixed together and a chemical reaction occurs, resulting in the formation of an insoluble compound.
• Precipitate


  A solid that forms during a chemical reaction between two solutions, indicating that a new insoluble compound has been created.