Qualitative Analysis by Eric Brown( CIE Chief Examiner of Paper 3)

WHEN ADDING ALKALI TO A SOLUTION THAT IS ACIDIC

No permanent precipitate will form until the solution has been neutralized.

If the alkali is added a little at a time a precipitate will form at the top of the solution where the acid in a

small volume of a solution has been neutralized.

The precipitate will disappear on shaking the test tube as mixing the solution restores acidity.

This is NOT a precipitate soluble in excess.

WHEN ADDING ACID TO A SOLUTION THAT IS ALKALINE

For Al, Pb and Zn the ion may be dissolved in an alkaline solution.

Adding an appropriate acid to the solution will produce a white ppt, soluble in excess of the acid. This

reversal of the “normal” test for colourless cations often causes confusion.

Gases

Gases that are coloured or have a distinctive smell

All are harmful

Ammonia – colourless

Chlorine- yellowish-green

Sulfur dioxide- choking smell

Nitrogen dioxide- brown coloured gas, is formed from colourless nitrogen oxide

Gases

Carbon dioxide- turn limewater chalky

Hydrogen- pop sound with lighted splinter

Oxygen- rekindle a glowing splinter

Density of gas Hydrogen<Oxygen<Carbon dioxide

Density ( at room temperature)

Air 1.20

Carbon dioxide 1.83

Hydrogen- 0.0833

Oxygen-1.33

ANIONS

Chromate

Two cations, Barium and Lead(II) ions, give yellow ppt. Compare the colour and brightness of the ppt.

Lighter colour- Barium chromate; Deeper colour- Lead chromate

Halides

With aqueous silver nitrate the halides give ppt.

Chloride- white ppt

Bromide- cream ppt

Iodide- yellow ppt

It can be difficult to tell these apart

Solubility of silver halide in dilute aqueous ammonia can help to identify the ion present

Halides

Dilute nitric acid is usually added before aqueous silver nitrate

The acid is added to remove interfering ions.

The hydroxide ions give a grey ppt of silver oxide

Carbonate ions give a white ppt

If the acid is added after the aqueous silver nitrate any ppt due to the interfering ions will disappear.