Qualitative Analysis

Question 1

Describe the carbonate ion test (+CO2 test)

Answer 1

• reacts with dilute nitric acid (HNO3)
• bubbles form
• CO2 test with limewater [Ca(OH)2] forms a cloudy solution containing CaCO3 white precipitate

Question 2

Describe sulfate test

Answer 2

• barium nitrate(Ba(NO3)2 is used to form BaSO4, an insoluble sulfate which will form a white precipitate
• it is used so no new ions e.g. Halide ions will be introduced to the solution

Question 3

Describe halide tests

Answer 3

• silver nitrate forms insoluble silver halides with different coloured precipitates.
• aqueous ammonia is used to distinguish the precipitates by testing precipitate solubility

Question 4

How soluble are the halide precipitates?

Answer 4

Cl- most soluble precipitate [soluble in dilute NH3(aq)]
Br- second most soluble precipitate [soluble in conc NH3(aq)]
I- least soluble precipitate [insoluble in conc NH3(aq)]

Question 5

What is the sequence of anion qualitative tests?

Answer 5

1. Carbonate - neither sulfate/halide ions produce bubbles with dilute acid
2. Sulfate - BaCO3 is also insoluble. Carrying a sulfate test on a carbonate will also form a white ppt
3. Halide - last because AgNO3 can form other insoluble solutions with ppts e.g. AgCO3 and Ag2SO4

Question 6

How to test for an ammonium ion?

Answer 6

• reacting an ammonium ion with a hydroxide ion with heat forms ammonia gas
• the gas can be tested using moist pH indicator paper which will turn blue with the presence of the alkaline ammonia gas