Tests For Cations

• What are cations?


  Positively charged ions
• What is the principle behind flame tests for identifying metal ions?


  • Compounds of metals burn with characteristic colors
  • Heating the substance reveals the color of the flame
• What color flame does lithium (Li⁺) produce in a flame test?


  Red
• What color flame does sodium (Na⁺) produce in a flame test?


  Yellow
• What color flame does potassium (K⁺) produce in a flame test?


  Lilac
• What color flame does calcium (Ca²⁺) produce in a flame test?


  Orange-red
• What color flame do copper ions (Cu²⁺) produce in a flame test?


  Blue-green
• What is the procedure for conducting a flame test?


  1. Clean a platinum wire loop with dilute HCl
  2. Hold the loop in a flame until it burns without color
  3. Dip the loop into the sample and place it back in the flame
• How do some metals form a colored precipitate with NaOH?

  1. Many metal hydroxides are insoluble and precipitate out of solution
  2. Add sodium hydroxide to a solution of the mystery compound
  3. Observe the color of the precipitate to identify the metal ion
• What color precipitate does copper(II) (Cu²⁺) form with NaOH?

  Blue
• What color precipitate does iron(II) (Fe²⁺) form with NaOH?

  Sludgy green
• What color precipitate does iron(III) (Fe³⁺) form with NaOH?


  Reddish brown
• How can you test for the presence of ammonia gas (NH₃)?

  1. Ammonia is smelly and harmful to eyes
  2. Use damp red litmus paper
  3. If ammonia is present, the paper turns blue
• How can you test for ammonium ions (NH₄⁺)?


  Add sodium hydroxide to the solution and check for ammonia gas
• What precautions should be taken when testing for ammonia gas?


  • Avoid smelling ammonia deliberately
  • It can be harmful to your eyes
• What is the role of damp litmus paper in testing for ammonia?


  • It allows ammonia gas to dissolve
  • Causes a color change to indicate presence