3.1.4 Qualitative analysis Flashcards
What are anions also know as?
Negative ions
How can you test for carbonate ions, CO₃²⁻?
Add strong acid to the sample
Collect the gas produced
Pass through lime water
What are the observations for a positve test for carbonate ions, CO₃²⁻
Fizzing
Limewater turns cloudy
Write an equation for the carbonate ion test
CO₃²⁻ (aq) + 2H⁺ (aq) -> H₂O (aq) + CO₂ (g)
How can you test for sulfate ions, SO₄²⁻?
Add dilute hydrochloric acid and barium chloride to the sample
What are the observations for a positive test for sulfate ions, SO₄²⁻?
White precipitate of barium sulfate is produced
Write an equation for the sulfate ion test
Ba²⁺ (aq) + SO₄²⁻ (aq) -> BaSO₄ (s)
What do you use to test for halide ions?
Acidified AqNO₃
Why do you add HNO₃ to test for halide ions and why not HCl?
To remove CO₃²⁻
Adding HCl would add Cl⁻ ions giving a false positive result
How can you test for a halide ion?
- 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
Write the result and equation for Cl⁻ test
White precipitate, soluble in dilute aqueous ammonia
Ag⁺ (aq) + Cl⁻ (aq) -> AgCl (s)
Write the result and equation for Br⁻ test
Cream ppt, soluble in concentrated aqueous ammonia only
Ag⁺ (aq) + Br⁻ (aq) -> AgBr (s)
Write the result and equation for I⁻ test
Yellow precipitate, insoluble in concentrated and dilute aqueous ammonia
Ag⁺ (aq) + I⁻ (aq) -> AgI (s)
When testing for carbonate, sulfate and halide ions, in which order should the tests be carried out and why?
- Carbonate test
- Sulfate test
- Halide test
Because barium ions form insoluble precipitate of BaCO₃ and silver ions form insoluble precipitate of Ag₂SO₄
What are cations also known as?
Positive ions
