Practical 4

• How do you test for group 2 cations?

  add sodium hydroxide

  the solubility of group 2 hydroxides increases down the group

  M2+ + 2OH- —> M(OH)2
• Give some practical consideration when testing for group 2 cations

  Practical considerations:

  • use calcium bromide instead of calcium chloride. because it’s safer. Calcium chloride is an irritant
  • avoid filling test tubes over halfway to prevent the contents from spillage
• Give another test for group 2 cations

  add dilute sulfuric acid

  Group 2 cations react with sulfuric acid to form metal sulfate.

  Mcl2 + H2SO4 —> MSO4 + 2HCl

  the solubility of group 2 sulfate decreases down group 2
• how do you test for ammonium ions ?

  Add sodium hydroxide to ammonium chloride and warm gently in water Bath. Hold a piece of red litmus paper in the mouth of the test tube.

  damp red litmus paper turn blue.

  NH4+ + OH- <=> NH3(g) + H2O

  ammonia is alkali, it reacts with the water on the litmus paper the reverse reaction occurs and ammonium and hydroxide ions are formed, hydroxide ions change the colour to blue.
• Practical consideration when testing for ammonium ions

  Heating speeds up reaction, ammonia may evaporate

  water Bath allows more gentle heating than a bunsen burner

  litmus paper must be damp so the reaction between ammonia and water can take place
• Testing for hydroxide ions 


  Dip universal indicator paper into the liquid

  universal indicator paper turns dark blue
• Testing for carbonate ions

  Place sodium carbonate in test tube and connect with a delivery tube to a test tube containing calcium hydroxide solution (Limewater). Add dilute hydrochloric acid to the test tube containing sodium carbonate.

  observation - sodium carbonate solution bubbles. Calcium hydroxide solution turns cloudy.

  CO3^2- + H+ —> CO2 + H2O

  Ca(OH)2 + CO2 —> CaCO3 + H2O

  Calcium carbonate insoluble in water so forms white precipitate.
• How do you test for the presence of sulfate ions

  Place magnesium sulfate into a test tube and add HCl and barium chloride.

  white precipitate forms

  SO4^2- + Ba2+ —> BaSO4(s)

  Barium sulfate is insoluble in water so forms a white precipitate.
• Practical consideration for testing the presence of sulfate ions

  Acidification (HCl) neutralises any carbonate ions

  2H+ + CO3^2- —> H2O + CO2