Qualitative Analysis - Practical (PDO)

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Record observations

 

Prerequisite Knowledge

• Test for ions and gases + expected observations

 

Notes

 

There are many observations! How do I know which one to record?

For each test, you should write down (wherever possible):
(1) what can the test be used to identify?
(2) what are the observations that you should look out for?

e.g. add nitric acid followed by barium nitrate.
(1) test for sulfate ions.
(2) look out for white ppt formed.

For a particular test, you should only write down the observations that you are supposed to look out for.

observations: white ppt formed [deduction: sulfate ion present] OR
observations: no ppt formed [deduction: sulfate ion not present]

 

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Test for Gases

• Whenever a gas is produced, write the following 4 points:
  (1) effervescence
  (2) colour and smell of gas (e.g. colourless, odourless gas produced)
  (3) gas test and observations (e.g. gas extinguished lighted splint with pop sound)
  (4) identity of gas (e.g. gas is hydrogen gas)
• Include ‘moist or damp’ when describing litmus paper colour change (e.g. moist red litmus paper turned blue)
• When describing colour change, state both the initial and final colour (e.g. purple potassium manganate (VII) solution turned from purple to colourless)

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Test for Cations

• What observations do we need to write down in our answers?
  Write obsevations specific to what is the procedure testing for.
  e.g. addition of NaOH is to test for cations; so we need to write down (1) whether ppt formed? (2) colour of ppt? (3) whether ppt dissolved in excess.
• When a ppt dissolves in excess reagent, need to state the colour of the resulting solution. e.g. white ppt dissolved in excess NaOH to form a colourless solution.
• When no ppt formed, write either ‘no visible change’ or ‘no ppt formed’.

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Test for Oxidising/ Reducing agents (OA/RA)

• Since OA and RA will give a colour change on undergoing reaction, we need to record the colour change for both the reagent and the solution separately
  e.g. add H⁺/KMnO₄ to a solution containing Fe²⁺; KMnO₄ turned from purple to colourless; solution turned from pale green (colourless) to yellow.