Unit 7 - Analysis
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Test for ammonia
• it has a pungent smell • turns damp red litmus paper blue • with the stopper of concentrated HCl it gives a white smoke (ammonium chloride)
Test for Cu++ (copper)
Dissolve compound in water and add dilute sodium hydroxide. If test is positive, a pale blue precipitate will be formed. Cu++ + 2OH- –> Cu(OH)2
Test for Fe++ (iron II) ions
Dissolve compound in water and add dilute sodium hydroxide. If test is positive, a moss green precipitate will be formed. Fe++ + 2OH- –> Fe(OH)2
What happens when you dissolve potassium, sodium, or ammonium ions in sodium hydroxide
There is no precipitate as these hydroxides are soluble in water
Test for Cl- (chloride) ions
Dissolve the compound in water. Add aqueous silver nitrate and dilute aqueous nitric acid. If test is positive a white precipitate will form. Ag+ + Cl- –> AgCl
How to perform a flame test
Use a clean platinum wire Dip it in concentrated hydrochloric acid Dip it in powdered compound View the colour of the flame
Ion gives a brick red flame in a flame test
Ca++ (calcium)
What colour gives an intense yellow flame in a flame test
Na+ (sodium)
What ion gives a green (blue centre) flame in a flame test
Cu++ (copper)
What ion gives a lilac flame in a flame test
K+ (potassium)
What ion gives a bright red flame in a flame test
Li+ (lithium)
Test for ion Br- (bromide)
Dissolve the compound in water. Add aqueous silver nitrate and dilute aqueous nitric acid. If test is positive a cream precipitate will form (silver bromide)
Test for I- (iodide) ions
Dissolve the compound in water. Add aqueous silver nitrate and dilute aqueous nitric acid. If test is positive a primrose yellow precipitate will be formed (silver iodide)
Test for the anion SO4 – (sulfate)
Dissolve the compound in water and add aqueous barium chloride solution (and HCl to remove carbonates). BaSO4 is insoluble in water so a white precipitate will form.
Test for anion CO3 – (carbonate)
Add dilute HCl. If there is effervescence it is either CO2 or H2. If the gas is CO2 then a carbonate is present CO3 – + 2H+ –> CO2 + H2O
Test for hydrogen
Lit splint in gas gives a squeaky pop
Hazards of hydrogen
It’s highly flammable (could explode)
Test for oxygen
Glowing splint relights in gas
Hazards of ammonia
It’s poisonous
Test for carbon dioxide
Bubble gas through limewater, it goes milky (goes clear again after a while)
Test for chlorine gas
Damp litmus paper is bleached white
Hazards of chlorine
It’s highly poisonous
Test for water vapour
Anhydrous copper sulfate turns from white to blue
Test for NH4+ (ammonium)
Dissolve the compound in an alkali (sodium hydroxide) and heat it. On heating, ammonia is less soluble, so it is released. NH4+ (s) + OH- (aq) –> NH3 (g) + H2O (g)