Analysis Tests

How to test for cations and anions and the results.

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Cations anre metal ions with a posative charge e.g. Cu or Fe3

Anions are non-metal ions and have a negative charge e.g. Cl- or SO32-

A range of chemical tests can be done to identify the anions and cations in a substance.

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Cation Tests

Tests with sodium hydroxide - NaOH - OH

For Cu2+: An insoluble (in excess NaOH) blue precipitate forms
Cu2+(aq)2OH(aq) --> Cu(OH)2(s)

For Fe2: An insoluble grey/green precipitate. After several minutes it oxidises to a rust coloured precipitate and iron (III) hydroxide.
Fe2+(aq)2OH-(aq) --> Fe9(OH)2(s)

For Fe3: An insoluble rust coloured precipitate forms.
Fe3+(aq)3OH-(aq) --> Fe(OH)3(s)

For Al3: A SOLUBLE white precipitate forms.
Al3+(aq)3OH-(aq) --> Al(OH)3(s)

For Ca2: An insoluble white precipitate forms
Ca2+(aq) +2OH-(aq) --> Ca(OH)2(s)

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NH4+ can be tested for with sodium hydroxide - NaOH

It must be warmed and then any gas produced should turn indicatior paper blue.

NH4+(aq)OH-(aq) --> NH3(aq) H2O(l)

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Flame Tests

Copper --> Blue (like church rooves)

Calcium --> brick red/ orange (you pick up a brick with your teeth)

Sodium --> yellow/orange (you sprinkle salt on the yolk of your egg)

Potassium --> lilac (Potassium is Purple)

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Anion Tests

To test for the halogens: use nitric acid - HNO3 and SILVER NITRATE - AgNO3

For Cl-: a white precipitate forms
Ag+(aq) + Cl-(aq) --> AgCl(s)

For Br-: A cream precipitate forms
Ag+(aq)Br-(aq) --> AgBr(s)

For I-: A yellow precipitate forms
(aq)+I-(aq) --> AgI(s)

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Anion Tests

To test for carbonates - CO32- - Use hydrochloric or nitric acid - HCl or HNO3
There is fizzing and a colourless gas given off (CO2 - test with limewater)
2H+(aq)CO32-(aq) --> CO2(g) + H2O(l)

To test for sulphItes SO32- - Use WARM HCl and test potassium dichromate solution
There is fizzing and a colouless pungent gas (SO2 Sulphur Dioxide) The potassium dichromate turns green/blue.
2H(aq)SO32-(aq) --> SO2(g) +H2O(l)

To test for sulphAtes SO42 Use nitric acid HNO3 and BARIUM CHLORIDE BaCL2
A white precipitate forms
(aq) --> BaSO4(s)

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