[8.3] qualitative analysis

Question 1

what is observed when AgNO₃ (aq) is added to chloride, bromide, and iodide ion solutions, after adding HCl?

Answer 1

• chloride = white precipitate (AgCl (s), silver chloride)
• bromide = cream precipitate (AgBr (s), silver bromide)
• iodide = yellow precipitate (AgI (s), silver iodide)

Question 2

what is observed when NH₃ (aq) is added to the silver halides?

Answer 2

• silver chloride precipitate dissolves in dilute ammonia solution
• silver bromide precipitate dissolves in concentrated ammonia solution, but not in dilute ammonia solution
• silver iodide precipitate does not dissolve in dilute or concentrated ammonia solution

> solubility of silver halides decreases going down the group

Question 3

what is the general ionic equation for testing for halides?

Answer 3

Ag⁺ (aq) + X⁻ (aq) -> AgX (s)

Question 4

what is the sequence of the qualitative tests for ions?

Answer 4

1. carbonates
2. sulfates
3. halides

Question 5

why must the carbonate test be carried out first?

Answer 5

• barium carbonate is insoluble
• this means that barium nitrate would give a white precipitate with carbonates as well as with sulfates, so the carbonate must be eliminated first

Question 6

why must the sulfate test be carried out before the halide test?

Answer 6

• silver sulfate is insoluble
• this means that silver nitrate would give a white precipitate with sulfates as well as with halides, so the sulfate must be eliminated first

Question 7

what is the test and observation for carbonates?

Answer 7

• test: add a few drops of dilute nitric acid
• observation: effervescence due to carbon dioxide gas

Question 8

what is the equation for bubbling carbon dioxide gas through limewater?

Answer 8

CO₂ + Ca(OH)₂ -> CaCO₃ (s) + H₂O

• Ca(OH)₂ is limewater
• CaCO₃ is white precipitate which gives the cloudy colour

Question 9

what is the test and observation for sulfates?

Answer 9

• test: add a few drops of nitric acid and an equal volume of aqueous barium nitrate
• observation: a white precipitate of barium sulfate

Question 10

what is the full and ionic equation for the sulfate test?

Answer 10

• full: MgSO₄ (aq) + BaCl₂ (aq) -> MgCl₂ (aq) + BaSO₄ (s)
• ionic: Ba²⁺ (aq) + SO₄²⁻ (aq) -> BaSO₄ (s)

Question 11

what is the test and observation for ammonium ions (NH₄⁺)?

Answer 11

• test: warm with dilute NaOH and test ammonia gas given off with damp red litmus paper
• observation: ammonia gas turns red litmus paper blue

Question 12

what is the ionic equation for the ammonium ion test?

Answer 12

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