3.1.4 Qualitative Analysis

Question 1

What are the flame test results?

Answer 1

Lithium - red
Sodium - yellow
Potassium - lilac
Calcium - brick red
Copper - blue

Question 2

How to carry out a flame test?

Answer 2

• dip nichrome wire into concentrated HCl and hold in blue Bunsen flame to clean
• dip wire back into acid and then powder being tested
• hold in blue flame to see colour

Question 3

How to use NaOH to find positive ions?

Answer 3

Most metal hydroxides are insoluble and transition metal ions form coloured precipitates when mixed with NaOH
Add a few drops of NaOH to solution being tested

Question 4

What are the results of the NaOH test for cations?

Answer 4

Copper (II) - blue
Iron (II) - green
Iron (III) - brown
Group 1 - no precipitate
Group 2 - white precipitate
Aluminium - white precipitate but disappears with excess NaOH

Question 5

What is an example ionic equation for the NaOH test?

Answer 5

Cu+2 + 2OH- -> Cu(OH)2

Question 6

What is the test for carbonate ions?

Answer 6

CO3 2- by reaction with H+ (aq) forming CO2 (g)

• Add dilute acid to solution and bubble gas formed through limewater
• Fizzing is seen, gas turns limewater cloudy so must be CO2

Question 7

What is an example equation for the carbonate test?

Answer 7

ZnCO3 +2HNO3 -> ZnNO3 + CO2 + H2O

Question 8

What is the test for sulfate ions?

Answer 8

• add dilute HCl followed by barium chloride solution to unknown solution
• a white precipitate of barium sulfate is formed
• HCl is added so that it reacts with any carbonate ions present avoiding a false positive. Using H2SO4 could also give a false positive by adding sulfate ions

Question 9

Ionic equation for sulfate test

Answer 9

Ba+2(aq) + SO4 -2(aq) -> BaSO4 (s)

Question 10

What is the test for halide ions?

Answer 10

• add dilute nitric acid followed by silver nitrate solution
• HCl will give Cl- ions and a false positive
• HNO3 reacts with any carbonate ions and removes them
• silver halide precipitate forms
  E.g. Ag+ + Cl- -> AgCl

Question 11

What are the results of the halogen tests?

Answer 11

Cl-
- white precipitate
- dissolves in dilute NH3

Br-
- cream precipitate
- dissolves in conc NH3

I-
- yellow precipitate
- doesn’t dissolve in NH3

Question 12

What is the test for ammonium (cation)?

Answer 12

• add aqueous NaOH and warm the mixture gently
• ammonia gas released has a pungent smell and will turn MOIST RED LITMUS PAPER BLUE

Question 13

Test for hydrogen

Answer 13

Alighted spill will go out with a squeaky pop

Question 14

Test for oxygen

Answer 14

A glowing spill relights

Question 15

Test for carbon dioxide

Answer 15

Limewater turns cloudy when CO2 is passed through it

Question 16

What is the sequence that tests for anions must be carried out in?

Answer 16

1. Carbonate
2. Sulfate
3. Halide