Module 3.1.4 - Qualitative analysis

Question 1

What does CaSH stand for?

Answer 1

Carbonate
Sulfate
Halide

Question 2

What is the test for carbonates?

Answer 2

Add dilute acid (HNO3) to the unknown solution
- positive: effervescence as CO2 gas is formed

Question 3

What is the ionic equation for positive carbonate test?

Answer 3

CO3 2- (aq) + 2 H+ (aq) -> H2O (l) + CO2 (g)

Question 4

How do you test for sulfate?

Answer 4

Add dilute HNO3 and then barium nitrate Ba(NO3)2
- positive: white precipitate forms

Question 5

What is the ionic equation for positive sulfate test?

Answer 5

Ba2+ (aq) + SO4 2- (aq) -> BaSO4 (s)

Question 6

What is the test for halide ions?

Answer 6

• Add aqueous silver nitrate solution (AgNO3) to sample of aqueous halide ion until precipitate forms
• Record colour of precipitate
• Test solubility of precipitate with dilute and concentrated ammonia solution

Question 7

What is the ionic equation for positive halide test?

Answer 7

Where X is halide:

Ag+ (aq) + X- (aq) -> AgX (s)

Question 8

What is the test for ammonium ions?

Answer 8

Add warm dilute aqueous sodium hydroxide to solution
- Positive: Ammonia gas produced when solution is warmed, detected with damp red litmus paper turning blue

Question 9

What is the ionic equation for positive ammonium test?

Answer 9

NH4+ (aq) + OH- (aq) -> NH3 (g) + H2O(l)

Question 10

Why should carbonates be tested first?

Answer 10

Only carbonate produce CO2(g) with an acid, no bubbling no carbonate

Question 11

Why should sulfates be tested second?

Answer 11

• Barium carbonate insoluble in water and forms a white precipitate also

Question 12

Why should halides be tested third?

Answer 12

Silver carbonate and silver sulfate (Ag2CO3 and Ag2SO4) are insoluble and will form precipitates

Question 13

What should you do to ensure all tests are completed before moving on?

Answer 13

Carbonate: Add acid until no more effervescence
Sulfate: Add excess of Ba(NO3)2 and filter solution to remove precipitate
Halide: Carry halide test out on filtrate then use ammonia solution to confirm halide identity