Qualitative analysis - cation test demystified

How do the precipitates look like?

A picture tells a thousand words.
Tip: My students will save these 2 tables in their phones for reference.

For white precipitates

For coloured precipitates

What should we record for cation test observations?

• If a precipitate is formed, record its colour. ->  A white precipitate is obtained.
  
• If there seems to be no reaction, record as -> No visible reaction (DO NOT write "No reaction")
  
• When you are asked to add more reagent until in excess, record what happens to the precipitate.

1. The white precipitate dissolves to form a  colourless solution.
2. The white precipitate is insoluble in excess reagent.

How do we memorise everything?

Although a simplified QA table is given during practical exam, I will always make my students understand this table thoroughly before sitting for the practical exam. If a student requires the QA table during the exam, it only means that he is not ready or confident enough.

Tips: Remember the colours for coloured precipitates first. Followed by white precipitates. Then those with no precipitates.

Many students memorise the table above but do not understand the concept behind. Have you ever wondered why does Ca2+ ions form a precipitate with aqueous sodium hydroxide but not with aqueous ammonia?

Calcium hydroxide is slightly soluble in water and would require a high concentration of hydroxide ions to react with it to form a precipitate.  Hence, since sodium hydroxide is a strong alkali, the calcium ions react with the high concentration of hydroxide ions to form a precipitate. Aqueous ammonia being a weak alkali do not have sufficient hydroxide ions and thus, no precipitate is seen.

Why does the white precipitate dissolve in excess aqueous sodium hydroxide to form a colourless solution for zinc ion, aluminium ion and lead(II) ion then?

The white precipitate is the hydroxide of zinc, aluminium and lead(II) which can react with aqueous sodium hydroxide to form soluble complex ions as they are amphoteric hydroxides.

Do put in extra effort for Qualitative Analysis. There is a chance that it may appear both in theory and practical exam!