Product Tests

Seite 1

Testing for Positive Ions

Flame tests can be used to identify metal ions, compounds of some metals burn with a characteristic colour, you simply put your substance in a flame and observe the colour of the flame:

Lithium, Li+, gives a crimson flame.

Sodium, Na+, gives a yellow flame.

Potassium, K+ gives a lilac fame.

Calcium, Ca2+, gives a red flame.

Barium, Ba2+, gives a green flame.

To test a compound in a lab you should use a clean wire loop and dip this into a sample of the compound and place it in the hottest part of the Bunsen flame. Make sure the loop is clear before using it by dipping it in hydrochloric acid and rinsing it with distilled water.

Some metal ions form a coloured precipitate when reacted with NaOH, many are insoluble and precipitate out of solution when formed some of which have a characteristic colour. You add a few drops of sodium hydroxide to a solution of the unknown compound, if you get a coloured insoluble hydroxide you can recognise the metal found in the compound.

Calcium, Ca2+, white precipitate - Ca2+ + 2OH- = Ca(OH)2

Copper (II), Cu2+, blue precipitate - Cu2+ + 2OH- = Cu(OH)2

Iron (II), Fe2+, green precipitate - Fe2+ + 2OH- = Fe(OH)2

Iron (III), Fe3+, brown precipitate - Fe3+ + 3OH- = Fe(OH)3

Aluminium, Al3+, white precipitate at first but redissolves in excess NaOH to form a colourless solution - Al3+ +3OH- = Al(OH)3

Magnesium, Mg2+, white precipitate - Mg2+ + 2OH- = Mg(OH)2

Seite 2

Testing for Negative Ions

You can test for carbonates by checking for the presense of CO2, this is done by:

1) Bubble limewater through it, if it is present then the limewater will become cloudy.

2) You can then use this to test for carbonate ions (CO3 2-), since carbonates react with dilute acids to form carbon dioxide.

acid + carbonate = salt + water + carbon dioxide

To test for halide ions such as chloride (Cl-), bromide (Br-) or iodide (I-) ions, add dilute nitric acid (HNO3), followed by silver nitrate solution (AgNO3).

A chloride gives a white precipitate of silver chloride - Ag+ + Cl- = AgCl

A bromide gives a cream precipitate of silver bromide - Ag+ + Br- = AgBr

An iodide gives a yellow precipitate of silver iodide - Ag+ + I- = AgI

To test for a sulphate ion (SO4 2-), add dilute HCl followed by barium chloride solution, BaCl2. A white precipitate of barium sulphate means that the original compound was a sulphate because Ba2+ + SO4 2- = BaSO.