Testing for 2 unknown cations in qualitative analysis is a common question tested in A Level Chemistry Practical, and it’s actually an easy- to- score question. So in this post, let’s look at how these questions look like, and why we are doing what we are doing. You will also find a flow chart on testing for 2 cations below.
The steps for testing for 2 cations usually goes this way:
- Add either aqueous sodium hydroxide or aqueous ammonia until no further change is seen
- Filter the mixture
- To the filtrate, add acid until no further change is seen.
Let’s go through in detail about testing of cations, and in particular, testing for 2 cations.
How do we know the qualitative analysis question involve testing for cations?
When asked to test for cations, students are often asked to add sodium hydroxide or ammonia solution until no further changes are seen. If you take a look at the qualitative analysis notes (this is provided during your practical exam), you will find that the tests for cations (in the A Level syllabus) involves adding either NaOH (aq) or NH3 (aq). So, if you are asked to add NaOH or NH3 until no further change is seen, chances are, that test is to test for cations.
So, how do we find out what cation that is? Well, initially when we add the alkali (i.e. NaOH or NH3), the purpose is to see whether there is a precipitate, and if there is, what is the colour of the precipitate. Next, we add the alkali in excess, and the purpose is to see whether the precipitate is soluble in excess. Based on whether there is a precipitate, colour of precipitate and whether the precipitate is soluble in excess NaOH or NH3, we can predict the cation present from the table below:
| Cations | Reaction with NaOH (aq) | Reaction with NH3 (aq) |
| Al3+(aq) | white ppt soluble in excess | white ppt insoluble in excess |
| NH4+ (aq) | ammonia produced on heating | – |
| Ba2+ (aq) | no ppt | no ppt |
| Ca2+ (aq) | white ppt when Ca2+ concentration is high (else no ppt) | no ppt |
| Cr3+ (aq) | grey-green ppt. soluble in excess giving dark green solution | grey-green ppt. insoluble in excess |
| Cu2+ (aq) | pale blue ppt. insoluble in excess | blue ppt. soluble in excess giving dark blue solution |
| Fe2+ (aq) | green ppt., turning brown on contact with air insoluble in excess | green ppt., turning brown on contact with air insoluble in excess |
| Fe3+ (aq) | red-brown ppt. insoluble in excess | red-brown ppt. insoluble in excess |
| Mg2+ (aq) | white ppt. insoluble in excess | white ppt. insoluble in excess |
| Mn2+ (aq) | off-white ppt., rapidly turning brown on contact with air insoluble in excess | off-white ppt., rapidly turning brown on contact with air insoluble in excess |
| Zn2+ (aq) | white ppt. soluble in excess | white ppt. soluble in excess |
How do we know the cation is not ammonium NH4+?
If you look at the cation table above, you will notice that testing for ammonium is different from the other cations. You will need to heat the solution. Hence, if you are not asked to heat the solution after adding sodium hydroxide, then it’s likely that ammonium ions are not present (since you are not asked to test for it).
Testing for 2 cations in Qualitative Analysis Practical
When you are asked to test for 2 cations, you will be asked to add NH3 or NaOH until not further change is seen. Initially, when you are adding these alkalis, you are precipitating the cations out. When added in excess, one of the cation will be soluble in excess and the other will not be (the won’t ask you to do a test if both precipitates are soluble in excess, or insoluble in excess, as you need to separate the 2 cations).
Once the 2 cations are separated, you will be able to see the colour of the filtrate and the reside clearly.
Question will then ask you to add acid to the filtrate. When acid is added, you can think of this step as removing the excess NaOH or NH3 that has dissolved the precipitate, and you will see a precipitate being regenerated. When you add excess acid, an acid- base reaction takes place (since the precipitate is a base). If the salt formed is soluble in water, then the ppt dissolves in excess.
I’ve summarized the testing of 2 cations in the flowchart below:
How to identify 2 cations with example from past qualitative analysis practical exam question
In the video below, I go through how to identify two cations. We also use a qualitative analysis practical exam question as an example. In addition, we also go through the chemical reactions that have taken place in this video.
Watch our other videos on A Level Chemistry here.
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