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 techniquesQuantity 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.

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

Limewater

Test solution1 cm

2 cm

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? E.g. if you are adding sodium hydroxide with

heating, what are you possibly testing for? E.g. if you are adding sodium hydroxide with

aluminium and with heating, what are you possibly testing for?

Metal cations

NH4+

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 Blue ppt soluble in excess NH4OH to give a deep blue solution

Fe2+ Dirty-green ppt insoluble in excess NaOH

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

(produces a white ppt with limewater)

The gas……