Question on remembering precipitates

[Stringy]

Hey guys.

Just wanted to ask, do I need to know the colours certain ions form when precipitating?
e.g. PO4 in NH3 followed by BaNO3 forms a white precipitate.
Or do i just need to know my solubility rules and what will precipitate out of solutions?

[KT Nyunt]

Hey guys.

Just wanted to ask, do I need to know the colours certain ions form when precipitating?
e.g. PO4 in NH3 followed by BaNO3 forms a white precipitate.
Or do i just need to know my solubility rules and what will precipitate out of solutions?

Yes, you would definitely need to know the colours as well as the solubility rules. Often questions would describe a list of precipitation reactions (so it would mention the colour of the ppt formed) and you would have to identify what anions/cations were present, and often the colour of the precipitate formed is the main identifying factor.
You would also have to describe methods to identify anions/cations in a solution. To differentiate between anions/cations with similar solubilities (and there are many), you would have to rely on the unique precipitate colours formed with certain anions and cations.

An example could be:
Copper, Iron (II) and Iron (III) all form precipitates with NaOH. So your solubility rules alone can't help you here. To differentiate between the three you need to know the colour of the ppt formed... copper (blue), Iron(II) (green) and Iron(III) (brown).

This is a hard task but you can often find some patterns which can help:
- precipitates with barium are white
- precipitates with copper are blue/green
- precipitates with silver are yellow/white
- most precipitates with iron are brown
etc

Hope this helps