3.1.4 - Qualitative Analysis
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What are anions also known as?
Negative ions
How can you test for carbonate ions, CO3 ^2-?
Add strong acid to the sample Collect gas produced Pass through lime water
What are the observations for a positive test of carbonate ions, CO3^2-?
Fizzing Limewater turns cloudy
Write an equation for the carbonate ion test
CO3^2- (aq) + 2H+ (aq) —-> H2O (aq) + CO2 (g)
How can you test for sulfate ions, SO4^2-?
Add dilute hydrochloric acid and barium chloride to the sample
What are the observations for a positive test of sulfate ions, SO4^2-?
White precipitate of barium sulfate is produced
Write an equation for the sulfate ion test
Ba^2+ (aq) + SO4^2- (aq) —–> BaSO4 (s)
What do you use to test for halide ions?
Acidified AgNO3
Why do you add HNO3 to test for halide ions and not HCl?
To remove CO3^2- Adding HCl would introduce 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 precipitate Soluble in concentrated aqueous ammonia only Ag+ (aq) + Br- (aq) ———> AgBr (s)
Write the result and equation for I- test
Yellow precipitate Insoluble in dilute and concentrated 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
How can you test for ammonium ions, NH4+
Add sodium hydroxide to the sample and warm it Test the gas produced with red litmus paper
What are the observations for positive ammonium ions test?
Red litmus paper turns blue Ammonia has a pungent smell
Write the equation for ammonium ions test
NH$+ (aq) + OH- (aq) —-> HN3 (aq) + H2O (aq)