Testing for ions (GCSE triple science)

brief explanation of how ions are identified.

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Cations and Anions


Metals usually form positive ions (called cations) by loosing electrons.

E.g. sodium atoms form positive ions when their atoms loose one electron.

Na -> Na+ + e-


Non-metals usually form negative ions (called anions) by gaining electrons.

E.g. chlorine atoms form negative ions by gaining one electron.

Cl2 + 2e- -> 2Cl-

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Flame tests

Flame Tests:

A clean piece of wire is dipped in a solution of a compound and then held in the hot part of a Bunsen flame, the colour produced can identify the ions present.

Different metals have different electron arrangements, with different energy levels, therefore they give out different colours of light.

Element Flame colour

Calcium (Ca2+) Brick red

Sodium (Na+) Yellow

Potassium (K+) Lilac

Copper (Cu2+) Green

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Testing for cations

Adding sodium hydroxide:

A precipitate is formed and can identify the ion present.

Cation Symbol Precipitate

Ammonium NH4+ None

Aluminium Al3+ White

Calcium Ca2+ White

Copper (II) Cu2+ Blue

Iron (II) Fe2+ Green

Iron (III) Fe3+ Brown (rust)

The precipitate will be a solid hydroxide of the cation.

E.g. CuSO4 + 2NaOH -> Cu(OH)2 + Na2SO4

copper(II)sulphate + sodium hydroxide -> copper(II) hydroxide + sodium sulphate

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Further tests

Further tests for ammonium, aluminium and calcium ions are needed.

Because aluminium and calcium precipitates are both white, and ammonium does not produce a precipitate.

Adding excess sodium hydroxide:

Calcium precipitate: not changed

aluminium precipitate: starts to dissolve

to test for ammonium ion:

heat the unknown with concentrated sodium hydroxide.

If the ions are present a smelly alkaline gas is given off (ammonia).

Ammonia also turns universal indicator paper blue.

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Testing for anions

Limus or universal indicator paper:

Turns blue/ purple when hydroxide ions (OH-) are present.

Adding hydrochloric acid:

Bubbles of gas are given off if carbonate ions (CO3,2+) os sulphate ions (SO3,2-)are present.

E.g. 2HCl + Na2CO3 -> 2NaCl + CO2 +H2O

Identifying the gas formed:

If the gas turns limewater milky, the gas is carbon dioxide, and carbonate ions are present.

If the gass is acidic and has a chocking smell, the gas is sulphurdioxide, and a sulphite was present.

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Testing for anions continued

Test for sulphate ions:

If barium chloride solution is added to a sulphate, a white precipitate of barium sulphate forms.

Carbonates and sulphates also produce a precipitate with barium chloride. So Add some hydrochloric acid:

sulphates: no effect

carbonates and sulphites: dissolves the white precipitate

The halide ions: (chlorine, bromide, iodide)

Add silver nitrate solution acidified with some dilute nitric acid.

Halide ion Precipitate

Chloride (Cl-) White

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Testing for anions continued 2

Halide Ion Precipitate

Bromide (Br-) Cream

Iodide (I-) Yellow

E.g. AgNO3 + NaCl -> AgCl + NaNO3

Silver nitrate + sodium chloride -> silver chloride (WHITE) + sodium nitrate

Dilute nitric acid needed to remove any carbonate, sulpite or hydroxide ions present.

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