TESTS FOR IONS & GASES
Gases
Cations
Anions
ammonia, NH3
- colorless alkaline gas
- strong sharp smell
test: hold damp red litmus paper in it
observation: litmus paper turns blue
& may notice the sharp smell
carbon dioxide, CO2
- colorless, weakly acidic gas
chlorine, Cl2
test: bubble the gas through limewater
- reacts with limewater (aqueous calcium hydroxide solution) to give white precipitate (calcium carbonate)
observation: limewater turns cloudy/milky
hydrogen, H2
- green poisonous gas
- bleaches dyes
test: hold damp paper indicator paper in the gas, in a fume cupboard
observation: indicator paper turns white
- colorless gas
- combines violently with oxygen when lit
test: collect the gas in a tube and hold a lighted splint to it
observation: the gas burns with a 'squeaky pop'
oxygen, O2
- colorless gas
- fuels burn more readily in it than air
test: collect the gas in a test-tube and hold glowing splint to it
observation: splint immediately ignites
sulfur dioxide, SO2
- colorless, poisonous, acidic gas
- choking smell
- will reduce potassium manganate(VII) ion to colorless potassium manganese(II) ions
test: soak a piece of filter paper in acidified aqueous potassium manganate(VII). place it in the gas
observation: color on the paper changes from purple to colorless
ammonium, NH4+
test: add dilute aqueous sodium hydroxide and heat gently
if cation is present: ammonia gas is given off
(turns litmus paper blue)
ionic equation:
NH4+ (aq) + OH- (aq) → NH3 (g) + H2O (l)
copper(II), Cu2+
test: add dilute aqueous sodium hydroxide
if cation is present: pale blue precipitate forms. adding more (excess) sodium hydroxide has no effect
ionic equation:
Cu2+ (aq) + 2OH- (aq) → Cu(OH)2 (s)
iron(II), Fe2+
test: add aqueous ammonia
halide ions
bromide ions, Br-
iodide ions, I-
chloride ions, Cl-
test: add an equal volume of dilute nitric acid to a small amount of the solution. then add aqueous silver nitrate
iodide ions, I-
chloride ions, Cl-
bromide ions, Br-
Ag+ (aq) + Cl- (aq) → AgCl (s)
Ag+ (aq) + Br- (aq) → AgBr (s)
Ag+ (aq) + I- (aq) → AgI (s)
if cation is present: a pale blue precipitate forms. it dissolves again if you add more (excess) ammonia, giving a deep blue solution
the precipitate dissolves again in excess ammonia because a soluble complex ion forms (ion with several negative ions / molecules around the positive metal ion
test: add dilute aqueous sodium hydroxide / aqueous ammonia
if cation is present: pale green precipitate forms
ionic equation:
Fe2+ (aq) + 2OH- (aq) → Fe(OH)2 (s)
iron(III), Fe3+
test: add dilute aqueous sodium hydroxide / aqueous ammonia
if cation is present: red-brown precipitate forms
ionic equation:
Fe3+ (aq) + 3OH- (aq) → Fe(OH)3 (s)
aluminium, Al3+
test: add dilute aqueous sodium hydroxide
if cation is present: white precipitate forms. it dissolves again if you add more (excess) sodium hydroxide, giving a colorless solution
the precipitate dissolves in excess sodium hydroxide because aluminium hydroxide is amphoteric (reacts with acids & alkalis)
ionic equation:
Al3+ (aq) + 3OH- (aq) → Al(OH)3 (s)
zinc, Zn2+
test: add aqueous ammonia
if cation is present: white precipitate forms. adding more (excess) ammonia has no effect
test: add dilute aqueous sodium hydroxide /aqueous ammonia
if cation is present: white precipitate forms in each case. in each case it dissolves again if you add more (excess) of the reagent, giving a colorless solution
the precipitate dissolves again in excess sodium hydroxide because zinc hydroxide is amphoteric.
it dissolves again in excess ammonia because a soluble complex ion forms
calcium, Ca2+
ionic equation:
Zn2+ (aq) + 2OH- (aq) → Zn(OH)2 (s)
test: add dilute aqueous sodium hydroxide
if cation is present: white precipitate forms. adding more (excess) sodium hydroxide has no effect
ionic equation:
Ca2+ (aq) + 2OH- (aq) → Ca(OH)2 (s)
test: add aqueous ammonia
if cation is present: no precipitate / very slight white precipitate
chromium, Cr3+
test: add dilute aqueous sodium hydroxide
if cation is present: grey-green precipitate forms.
it dissolves again if you add more sodium hydroxide, giving a green solution.
the precipitate dissolves in excess sodium hydroxide because chromium hydroxide is amphoteric
ionic equation:
Cr3+ (aq) + 3OH- (aq) → Cr(OH)3 (s)
test: add aqueous ammonia
if cation is present: grey-green precipitate forms. it dissolves again if you add more ammonia, giving a purple solution
the precipitate dissolves again in excess ammonia because a soluble complex ion forms
sulfate ions
test: add an equal volume of dilute HCl to a small amount of the solution. then add barium solution
yellow precipitate
white precipitate
cream precipitate
if sulfate ions are present: barium sulfte is insoluble, so a white precipitate will form
ionic equation:
Ba2+ (aq) + SO4 2- (aq) → BaSO4 (s)
sulfite ions
test: add an equal volume of dilute HCl acid to a small amount of the solution. heat the mixture gently
if sulfite ions are present: sulfur dioxide gas is given off
ionic equation:
SO3 2- (aq) + 2H+ (aq) → H2O (l) + SO2 (g)
nitrate ions
test for sulfur dioxide gas: soak a piece of filter paper in acidified aqueous potassium manganate(VII) [purple]. place it in the mouth of the test-tube. the color turn from purple → colorless
test: add a little dilute aqueous sodium hydroxide to a small amount of the solid or solution, then some small pieces of aluminium foil. heat gently
if nitrate ions are present: ammonia gas is given off
ionic equation:
8Al (s) + 3NO3- (aq) + 5OH- (aq) + 2H2O (l) → 3NH3 (g) + 8AlO2- (aq)
carbonate ions
test: add a little dilute HCl acid to a small amount of the solid or solution
if carbonate ions are present: the mixture bubbles and carbon dioxide gas is given off, turning limewater milky
ionic equation:
2H+ (aq) + CO3 2- (aq) → CO2 (g) + H2O (l)
- silver halides are insoluble. so if halide ions are present, a precipitate will form