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-
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
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
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.
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.
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
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.