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

A+special+test+for+oxygen!

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LAB.+TEST+FOR+AMMONIA+GAS.

sulfur dioxide gas test 1

sulfur dioxide gas test 2

gas-tests

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)

LAB.+TEST+FOR+CHLORINE+GAS.

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