2.37 describe tests for the cations:

i Li+, Na+, K+, Ca2+ using flame tests

Method:

  1. Dip platinum wire into (dilute) hydrochloric acid
  2. Place into raring flame until it no longer produces colour
  3. Place cleaned loop wire into HCl once more, then place in salt sample under test
  4. PLace wire in a blue, roaring bunsen flame
  5. Colour determines the cation present

Screen Shot 2016-06-07 at 8.05.41 AM

ii NH4 +, using sodium hydroxide solution and identifying the ammonia evolved

  • NH4 + OH- = NH3 + H2O
  • Ammonium ions + Hydroxide ions –> Ammonia + Water

Method:

  1. Add aquaeous sodium hydroxide ions to the solid and warm gently over a bunsen burner in a boiling tube
  2. Place damp (H2O) Red Litmus Paper
  3. If ammonia is present:

– Pungent smell

Red litmus is turns blue

  •  You must dampen the paper to allow the ammonia ions to dissolve into the paper.
  • Alkalis contain hydroxide ions – the hydroxide ions change the litmus paper to blue, the colour of alkalis

iii Cu2+, Fe2+ and Fe3+, using sodium hydroxide solution

EXAMPLE:

CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)

(blue solution + colourless solution → blue precipitate + colourless solution)

Screen Shot 2016-07-21 at 2.01.39 PM.png

Method:

  1. Add reagent (sodium hydroxide) one drop at a time
  2. Precipitate (Insoluble solid) should form alongside an aqueous solution.

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