Qualitative Analysis (testing for cations, anions and gases)

Test for aqueous cations

If you would like to find out how a method to memorize the cation table, click here.


Cations
Effect of adding aqueous sodium hydroxide
Effect of adding aqueous ammonia
Aluminium (Al3+)
White ppt.
Soluble in excess sodium hydroxide to give a colourless solution.
White ppt.
Insoluble in excess aqueous ammonia.
Ammonium (NH4+)
Not ppt.
When heated with sodium hydroxide solution, ammonia gas is evolved.
No ppt.
Calcium (Ca2+)
White ppt.
Insoluble in excess sodium hydroxide.
No ppt.
Copper (II) (Cu2+)
Blue ppt.
Insoluble in excess sodium hydroxide.
Blue ppt.
Soluble in excess aqueous ammonia to give a deep blue solution.
Iron (II) (Fe2+)
Green ppt.
Insoluble in excess sodium hydroxide.
Green ppt.
Insoluble in excess aqueous ammonia.
Iron (III) (Fe3+)
Reddish- brown ppt.
Insoluble in excess sodium hydroxide.
Reddish- brown ppt.
Insoluble in excess aqueous ammonia.
Lead (II) (Pb2+)
White ppt.
Soluble in excess sodium hydroxide to give a colourless solution.
White ppt.
Insoluble in excess aqueous ammonia.
Zinc (Zn2+)
White ppt.
Soluble in excess sodium hydroxide to give a colourless solution.
White ppt.
Soluble in excess ammonia to give a colourless solution.





Test for Anions
anion
Distinguishing test and result
Ionic equation
carbonate (CO32-)
Add acid. Carbon dioxide gas is produced.
2H+ (aq)+ CO32- (aq) ----> H2O (l) + CO2 (g)
(provided that the carbonate is a soluble one)
chloride (Cl-)
Acidify with nitric acid, then add aqueous silver nitrate.
White ppt formed.
OR
Acidify with nitric acid, then add lead (II) nitrate.
White ppt formed.
Ag + (aq) + Cl- (aq) ---> AgCl (s)



Pb 2+ (aq) + 2Cl- (aq) ---> PbCl2(s)

iodide (I-)
Acidify with nitric acid, then add aqueous silver nitrate.
Yellow ppt formed.
OR
Acidify with nitric acid, then add lead (II) nitrate.
Yellow ppt formed.
Ag + (aq) + I- (aq) ---> AgI (s)



Pb 2+ (aq) + 2I- (aq) ---> PbI2(s)

nitrate (NO3-)
Add aqueous sodium hydroxide, then aluminium foil and warm carefully. Ammonia gas produced.
 -
sulfate (SO42-)
Acidify with dilute nitric acid and then add aqueous barium nitrate.
White ppt formed.
OR
Acidify with dilute hydrochloric acid and then add aqueous barium chloride.
White ppt formed.
Ba2+ (aq)+ SO42-(aq) --> BaSO4(s)

Test for Gases
- If gas is produced from a solution, describe it this way:


                Effervescence of (colour), (smell) gas, (effect on damp litmus paper), distinguishing test.
                e.g. Effervescence of colourless, odourless gas, has no effect on both damp blue and red     
                      litmus paper. Relights a glowing splint. [ for oxygen]

-  If a gas is produced from a solid, you will not see any bubbles, so describe this way:
               (colour), (smell) gas, (effect on damp litmus paper), distinguishing test.

Gas
Distinguishing test and result
Effect on damp litmus paper
Ammonia (NH3)
Turns damp red litmus paper blue.
(see distinguishing test and result)
Carbon dioxide (CO2)
Forms a white ppt with calcium hydroxide solution (or limewater).
Turns damp blue litmus paper red.
Chlorine gas (Cl2)
Bleaches litmus paper.
Turns damp blue litmus paper red and bleaches it.
Hydrogen (H2)
Extinguishes a lighted splint with a 'pop' sound.
Has no effect on both blue and red damp litmus paper.
Oxygen (O2)
Relights a glowing splint.
Has no effect on both blue and red damp litmus paper.
Sulfur dioxide (SO2)
Turns aqueous potasssium dichromate (VI) from orange to green.
Turns damp blue litmus paper red (note SO2 also can bleach litmus paper).

15 comments:

  1. This is amazingly helpful! I've been looking everywhere for this! Thanks!

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  2. Thanks a lot. very helpful. There is a mistake though with the effect of aqueous ammonia on copper(2) .

    ReplyDelete
  3. Thank you Andy for pointing out the mistake. It has now been amended! =)

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    Replies
    1. Hi Nayra, it's great that you find these helpful.

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  5. Very helpful thank you....:)

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  8. Replies
    1. Thanks Muhammad!

      We also have questions on Qualitative analysis on this website. You may like to try them on :

      http://chemistryforsg-olevel.blogspot.com/search/label/qualitative%20analysis

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