M3 B4 Qualitative Analysis
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Qualitative tests rely on observations. What may these observations be? (4)
• Bubbles • A precipitate • A colour change • Identification of a gas
All carbonates react with _________ to form _______ _________ gas.
All carbonates react with ACOIDS to form CARBON DIOXIDE gas.
Carbonate test: Reagent?
Any acid (nitric acid HNO3)
Carbonate test: Observation?
Effervescence/ bubbles/ fizzes
What is the equation for sodium carbonate and nitric acid?
2HNO3 (aq) + Na2CO3 (aq/ s) = 2NaNO3 (aq) + H2O (l) + CO2 (g)
What test do we use to confirm that this gas is carbon dioxide?
Pump the gas through limewater ( a saturated solution of Ca(OH)2 ). This will form a white precipitate of Calcium carbonate ( CaCO3 )
What is the equation of testing for carbon dioxide?
CO2 (g) + Ca(OH)2 (aq) = CaCO3 (s) + H2O (l)
Sulfate test – which ion is being tested?
Sulfate ion, SO₄²⁻
Why can sulfate ions be identified using barium ions?
Most sulfates are soluble, but barium sulfate (BaSO₄) is very insoluble and forms a precipitate
What reagent is used in the sulfate test?
A soluble barium compound
Give two reagents used in the sulfate test
Barium nitrate, Ba(NO₃)₂(aq), or barium chloride, BaCl₂(aq)
Why is barium nitrate preferred if halide tests are done afterwards?
It does not introduce halide ions into the solution
What is the positive observation in the sulfate test?
A white precipitate forms
What is the white precipitate formed in the sulfate test?
Barium sulfate, BaSO₄(s)
What does a white precipitate in the sulfate test indicate?
The presence of sulfate ions, SO₄²⁻
Write the full equation for barium nitrate reacting with sodium sulfate
Ba(NO₃)₂(aq) + Na₂SO₄(aq) → 2NaNO₃(aq) + BaSO₄(s)
Write the ionic equation for the sulfate test
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
What colour is barium sulfate?
White
What reagent is used to test for halide ions?
Silver nitrate solution, AgNO₃(aq), after acidifying with dilute nitric acid
Why must dilute nitric acid be added before testing for halides?
To remove carbonate ions that would otherwise form a precipitate with silver ions
Why is nitric acid used instead of hydrochloric acid?
Hydrochloric acid contains chloride ions which would interfere with the test
Ionic equation for the chloride test
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Observation for chloride ions (Cl⁻) with silver nitrate
White precipitate of silver chloride
Solubility of silver chloride in ammonia
Dissolves in dilute ammonia solution