Ch 11 Qualitative Analysis (qa)

Qualitative Analysis (QA) Identification of unknowns in Chemistry Practical 5068/3

Important QA techniques Cleanliness  Apparatus must be clean.  

Washed with distilled water. Should be dried using a bunsen flame if a solid needs to be introduced & tested.

Labeling  Label all test tubes / beakers to avoid confusion.  

Get & use a water-soluble marker. Be neat & organised.

Important QA techniques Quantity of reagents  Use no more than 1 cm depth of chemical or just enough to fill the hemisphere at the bottom of the test tube. 

Adding other chemicals to the test solution should be done slowly and in small amounts. Shake thoroughly after each addition.

Test solution



Limewater should be used sparingly when testing for carbon dioxide – ensure fast & accurate results.

1 cm 2 cm

Limewater

Important QA techniques Testing for gases  Read ahead and be prepared for the tests.  Make

intelligent guesses as to which test to carry out e.g. powdered substance with acid shouldn’t be tested with a lighted splint. ???

 Note

colour & smell of gases.

Important QA techniques Testing for gases  Conduct appropriate test & identify gas.    

Damp litmus papers Glowing/lighted splints Passing gas through limewater Etc….

 However, 

do not test for obvious “results”.

e.g. testing for ammonia gas (NH3) if you have added aqueous ammonia as a reagent.

Important QA techniques Recording observations  Record all observations as you are carrying out the experiments.  Any

colour changes.  Any ppt formed, soluble/insoluble in excess?  Any gases evolved; colour, smell, test, identify.  Any noise, heat or light observed/produced.  Any negative results.

Important QA techniques Recording observations  Record   

negative results e.g.

No precipitate (ppt) is formed No gas is evolved No visible reaction

Important QA techniques Recording observations  Be

aware of what is happening and why certain tests are being carried out.   

E.g. if you are adding sodium hydroxide, what are you possibly testing for? Metal cations E.g. if you are adding sodium hydroxide with heating, what are you possibly testing for? NH4+ E.g. if you are adding sodium hydroxide with aluminium and with heating, what are you possibly testing for? NO3−

Use of OH− ions to identify cations Reaction with NaOH Reaction with NH4OH Ca2+

White ppt insoluble in excess NaOH No visible reaction

Al3+

White ppt soluble in excess NaOH to form a colourless solution

White ppt insoluble in excess NH4OH

Pb2+

White ppt soluble in excess NaOH to form a colourless solution

White ppt insoluble in excess NH4OH

Zn2+

White ppt soluble in excess NaOH to form a colourless solution

White ppt soluble in excess NH4OH to form a colourless solution

Cu2+

Blue ppt insoluble in excess NaOH

Fe2+

Dirty-green ppt insoluble in excess NaOH

Blue ppt soluble in excess NH4OH to give a deep blue solution Dirty-green ppt insoluble in excess NH4OH

Fe3+

Reddish-brown ppt insoluble in excess NaOH

Reddish-brown ppt insoluble in excess NH4OH

NH4+

NH3 gas produced on warming turns damp red litmus to blue. Pungent smell.

No visible reaction

The gas…… (produces a white ppt with limewater)