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, CO₃²⁻?

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, CO₃²⁻?

Answer 3

• Fizzing
• Limewater turns cloudy

Question 4

Write an equation for the carbonate ion test.

Answer 4

CO₃²⁻ (aq) + 2H⁺ (aq) -> H₂O (aq) + CO₂ (g)

Question 5

How can you test for sulfate ions, SO₄²⁻?

Answer 5

• Add dilute hydrochloric acid
• Add barium chloride to the sample

Question 6

What are the observations for a positive test of sulfate ions, SO₄²⁻?

Answer 6

• White precipitate of barium sulfate is produced

Question 7

Write an equation for the sulfate ion test.

Answer 7

Ba²⁺ (aq) + SO₄²⁻ (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 halide ions and why not HCl?

Answer 9

• To remove CO₃²⁻
• 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 the 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 precipitate, 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 form insoluble precipitate of BaCO₃
   - Silver ions form insoluble 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 a 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 (l)