Test for Anions

The tests for anions are relatively more complicated as compared to those for the cations. But nonetheless, let's attempt to tackle them all in an organised manner.

If We're Classifying by Reactants Added...

• Point to note: To differentiate between carbonates and sulfates, it is important that you add barium nitrate or lead (II) nitrate first. Why? 
• The problem with carbonates is that they form precipitate with any acid added. But carbonates don't react with nitrates. That's why you should add barium or lead (II) nitrate first, because these would produce precipitates BaSO4 or PBSO4 which are insoluble.

If We're Classifying by Reactions...

So yeah, the most memorable traits of each anion:

• Carbonate — Insoluble with any acid added, CO2 gas produced
• Sulfate — White ppt with barium or lead (II) nitrate; soluble in nitric acid
• Chloride —  White ppt with Pb or Ag; forms white crystals when heated then cooled
• Iodide — Yellow ppt ftw
• Nitrate — Devarda's alloy, heating, pungent NH3 gas produced

Tests for Cations

To understand tests for cations, we must first know: What are cations? Cations are basically positively charged ions, if you still don't know what I'm talking about, check out my post on ions here.

To test for cations, you usually use aqueous sodium hydroxide or aqueous ammonia. To identify the cation, you look at three things:

• The colour of the precipitate formed  
• The solubility in excess alkali (base)
• Reactivity to a flame test (only if required for further identification)

COLOUR OF PRECIPITATE

When NaOH is combined with cations, they form metal hydroxides. Soluble metal hydroxides are all those in Group I and ammonia. Meanwhile, any other metal hydroxide is insoluble. Insoluble means? THEY'LL FORM A PRECIPITATE. <-- Great.

Adding NaOH to any salt solutions containing transition metals usually form coloured solutions, while salt solutions with other metal ions(except Group I and ammonia) form white precipitate.

For ammonia, however, since it has a lower concentration of hydroxide ions, slightly soluble hydroxides such as calcium and barium hydroxides are not precipitated. Can't remember your solubility table? Here it is:

SOLUBILITY IN EXCESS ALKALI/BASE

So there is this trio of metal hydroxides that are fortunately or unfortunately complicated. Aluminium (II) Oxide, Zinc (II) Oxide and Lead (II) Oxide are amphoteric in nature, meaning they react with both acids and bases. When this happens, they form complex salts that are soluble to give colourless solutions.

So what happens exactly when you add excess alkali (base)?

Acid-base neutralisation takes place to form salt and water. That's why when you add excess base, the white precipitate you saw would dissolve and form a colourless solution. 

For ammonia, only two metal ions form precipitates which then dissolve to form soluble complexes when excess ammonia is added: Zinc and Copper. Copper would form a deep blue solution whilst zinc would form a colourless solution.

REACTIVITY TO FLAME TEST

After all those troublesome things you did, you still have more?? Yeah.

Fortunately or unfortunately, this isn't covered in the syllabus, so this'll be for another post another day...To sum up, here is everything in a pretty table: