PAG 4- Testing for anions- group 7, hydroxide, carbonate, sulfate ions

Question 1

Carbonate ions test

Answer 1

• Add any dilute acid and observe effervescence
• Bubble gas through limewater (to test for CO2)- turns cloudy white

Question 2

Test for sulfate ions

Answer 2

• BaCl2 solution added- forms white precipitate
• Ba2+ ions should be added via Ba(NO3)2 if halide test to be done (as chloride ions become introduced if BaCl2 used)

Question 3

Test for halide ions

Answer 3

• Add nitric acid, reacts with any existing anions to prevent false results, then silver nitrate (AgNO3)
• Chloride ions- white precipitate
• Bromide ions- cream precipitate
• Iodide ions- yellow precipitate
• Add NH3 to precipitates for clearer results:
• Chloride precipitate- dissolves in dilute NH3
• Bromide precipitate- dissolves in concentrated NH3
• Iodide precipitate- insoluble in concentrated NH3

Question 4

Testing order and why

Answer 4

Carbonate, sulfate, halide
- Carbonate produces only bubbles
- If sulfate test done on carbonate, white precipitate is also formed, which becomes confusing

Question 5

Displacement reactions (halogen-halides)

Answer 5

• Solution of each halogen is added to aqueous solutions of the other two halides.
• Halogen displaces halide from solution, causing a colour change.
• Organic solvent (cyclohexane) can be added to observe colour change easily- the halogen dissolved in the organic solvent forms a layer above the aqueous layer, forming a coloured band.
• More reactive halogens displace less reactive halide ions (reactivity decreases going down group).
• KBr and Cl2- aqueous layer=yellow, organic layer=orange
• KI and Cl2- aqueous layer=brown, organic layer=purple
• KI and Br2- aqueous layer= brown, organic layer=purple