Qualitative analysis of ions

Question 1

Test for NH4+ ions

Answer 1

Warm with NaOH solution and test gas produced with moist red litmus. Turns blue.
Can also test gas with universal indicator paper. Will turn blue. The gas is alkaline, so its ammonia.
Can also test gas with aw glass rod dipped in conc. HCl. White fumes of ammonium chloride

Question 2

Test for halide ions

Answer 2

Make a solution. Acidify using nitric acid. Add silver nitrate solution.
Precipitates produced (White - chloride, cream - bromide, yellow - iodide)
AgCl - Dissolves in dilute aq. ammonia
AgBr - Dissovles in conc. ammonia
AgI - Not dissolved

Question 3

Test for sulfate ions

Answer 3

Acidify solution with dilute HCl or nitric acid. Add barium chloride solution. White ppt. formed

Question 4

Test for carbonate ions

Answer 4

Add dilute acid. Test gas produced with limewater. Turns limewater cloudy

Question 5

Why is nitric acid added to the halide ion test?

Answer 5

Removes other ions such as carbonate or hydroxide ions which may react with silver ions forming precipitates that would interfere with the test.

Question 6

Why is acid added to sulfate ion test?

Answer 6

To remove any carbonate ions as barium carbonate ppt. is indistinguishable from barium sulfate.

Question 7

Colour of Cu2+ in solution

Answer 7

Blue solution

Question 8

Colour of Mn2+ in solution

Answer 8

Pale pink solution

Question 9

Colour of Fe2+ in solution

Answer 9

Green solution

Question 10

Colour of Fe3+ in solution

Answer 10

Yellow/brown solution

Question 11

Colour of Cr3+ in solution

Answer 11

Green solution

Question 12

Colour of Cu(OH)2

Answer 12

Blue ppt.

Question 13

Colour of Mn(OH)2

Answer 13

Pale brown ppt.

Question 14

Colour of Fe(OH)2

Answer 14

Green ppt.

Question 15

Colour of Fe(OH)3

Answer 15

Brown ppt.

Question 16

Colour of Cr(OH)3

Answer 16

Green ppt.

Question 17

What happens to Mn(OH)2 when its left?

Answer 17

It oxidises to brown Mn2O3 in air

Question 18

What happens when you add excess NaOH after making Cr(OH)3

Answer 18

Redissolves and goes green again.

Question 19

What happens when you add excess ammonia to Cr?

Answer 19

Ligand exchange as NH3 and H2O are uncharged and similar in size. [Cr(NH3)6]3+ is formed, which is a purple solution

Question 20

What happens when you add excess ammonia to Cu?

Answer 20

Incomplete ligand exchange so it has 4 ammonias and 2 waters. Deep blue solution

Question 21

What is the sequnce of tests for anions?

Answer 21

Carbonate, sulfate, halide as barium carbonate and silver sulfate are both insoluble.