3.1.4 Qualitative analysis

Question 1

how do you test for halides

Answer 1

• add solution containing aqueous silver ions (silver nitrate solution)
• if halide present, an insoluble silver halide precipitate forms (they have diff colours)

Question 2

what is the general equation for the reaction of halides when testing for them

Answer 2

Ag+(aq) + X-(aq) —> AgX(s)

Question 3

when doing halide tests, what colour precipitates are formed for Cl2, Br2 and I2

Answer 3

• Cl2 = white
• Br2 = cream
• I2 = yellow

Question 4

how can you tell the different precipitate colours apart in halide tests

Answer 4

• add aqueous ammonia to test for solubility of precipitate
• chloride + bromide are soluble in dilute NH3
• Iodide is insoluble in conc NH3

Question 5

how do you test for carbonate ions

Answer 5

• add nitric acid
• effervescence observed as CO2 produced
• to prove gas is CO2, bubble it through limewater (Ca(OH)2) which will turn cloudy

Question 6

give the equation for CO2 with limewater

Answer 6

CO2(g) + Ca(OH)2(aq) —> CaCO3(s) + H2O(l)

Question 7

how do you tests for sulfate ions

Answer 7

• add solution containing aqueous barium ions (barium nitrate solution)
• if sulfate ions present, insoluble barium sulfate forms as a white precipitate

Question 8

give the ionic equation for the sulfate test

Answer 8

SO4 2- (aq) + Ba2+ (aq) —> BaSO4 (s)

Question 9

if you need to analyse an unknown organic compound, you need to carry out the tests for the anions in the correct order which is:

Answer 9

1. Carbonate, CO3 2-
2. Sulfate, SO4 2-
3. Halides, Cl-, Br-, I-

Question 10

how do you test for ammonium ions

Answer 10

• when heated together, ammonium ions + aqueous hydroxide ions react to form ammonia gas
• add aqueous hydroxide
• warm gently - ammonia gas will be released (alkali)
• damp red litmus paper will turn blue in presence of ammonia gas