3.1.4: Qualitative analysis

Question 1

What are anions also known as?

Answer 1

negative ions

Question 2

How can you test for carbonate ions, CO3 2-?

Answer 2

Add strong acid to the sample
Collect the gas produced
Pass through lime water

Question 3

What are the observations for a positive test of carbonate ions, CO3 2-?

Answer 3

Fizzing

Limewater turns cloudy

Question 4

Write an equation for the carbonate ion test

Answer 4

CO3 2- (aq) + 2H+ (aq) → H2O (aq) + CO2 (g)

Question 5

How can you test for sulphate ions, SO4 2-?

Answer 5

● Add dilute hydrochloric acid and

barium chloride to the sample

Question 6

What are the observations for a positive test of sulfate ions, SO4 2-?

Answer 6

White precipitate of barium sulfate is

produced

Question 7

Write an equation for the sulfate ion test

Answer 7

Ba2+ (aq) + SO4 2- (aq) → BaSO4 (s)

Question 8

What do you use to test for halide ions?

Answer 8

Acidified AgNO3

Question 9

Why do you add HNO3 to test for halides and why not HCl?

Answer 9

To remove CO₃ 2-

Adding HCl would add Cl- ions, giving a false positive result

Question 10

How can you test for a halide ion?

Answer 10

● Dissolve the sample in water
● Add aqueous silver nitrate
● Record the colour change
● If difficult to distinguish the colour, add aqueous ammonia,
first dilute ammonia then concentrated ammonia
● Note the solubility of precipitate

Question 11

Write the result and equation for Cl- test

Answer 11

White precipitate, soluble in dilute
aqueous ammonia
Ag+ (aq) + Cl- (aq)→ AgCl (s)

Question 12

Write the result and equation for Br- test

Answer 12

Cream ppt, soluble in concentrated
aqueous ammonia only
Ag+ (aq)+ Br- (aq)→ AgBr (s)

Question 13

Write the result and equation for I- test

Answer 13

Yellow precipitate, insoluble in
concentrated and dilute aqueous
ammonia
Ag+ (aq)+ I- (aq)→ AgI (s)

Question 14

When testing for carbonate, sulfate and halide ions, in which order should the tests be carried out and why?

Answer 14

1. Carbonate test
2. Sulfate test
3. Halide test
   Because barium ions forms insoluble precipitate of BaCO3 and silver ions form insolube precipitate of Ag2SO4

Question 15

What are cations also known as?

Answer 15

positive ions

Question 16

How can you test for ammonium ions, NH4+?

Answer 16

Add sodium hydroxide to the sample and warm it

Test the gas produced with red litmus paper

Question 17

What are the observations for positive ammonium ions test?

Answer 17

● Red litmus paper turns blue

● Ammonia has a pungent smell

Question 18

Write the equation for ammonium ions test

Answer 18

NH4+ (aq) + OH- (aq) → NH3 (aq) + H2O (aq)