In O Level Chemistry, students learn the qualitative analysis table of cations, anions and also testing for gases. Let’s look at these three groups of chemical tests and more in this post.
- qualitative analysis table of cations
- how to remember the cation table easily
- qualitative analysis table of anions
- testing for gases
- Qualitative analysis and more
Do note that for the practical exam (paper 3 for Pure Chemistry), the tables for testing of cations, anions and gases are provided (so no need to memorize them). However, students still need to know them for their theory paper (paper 1 and 2 for pure Chemistry).
Qualitative analysis table of cations
Here are the list of qualitative analysis of cations that are tested in O Level Chemistry.
Do note that since all tests involve precipitation, the test is for aqueous ions (i.e. the compound must be dissolved in water during the tests).
The following is not the exact table you will see in your data booklet. I have added in my notes as well, so that it is more useful for students. The portion that I’ve added are in italics.
| cation | effect of aqueous sodium hydroxide (NaOH) | effect of aqueous ammonia (NH3) |
| aluminium (Al3+) | white ppt., soluble in excess giving a colourless solution white ppt is Al(OH)3. | white ppt., insoluble in excess white ppt is Al(OH)3. |
| ammonium (NH4+) | ammonia produced on warming | – |
| calcium (Ca2+) | white ppt., insoluble in excess white ppt is Ca(OH)2. | no ppt. |
| copper(II) (Cu2+) | light blue ppt., insoluble in excess blue ppt is Cu(OH)2. | light blue ppt., soluble in excess giving a dark blue solution blue ppt is Cu(OH)2. |
| iron(II) (Fe2+) | green ppt., insoluble in excess green ppt is Fe(OH)2. | green ppt., insoluble in excess green ppt is Fe(OH)2. |
| iron(III) (Fe3+) | red-brown ppt., insoluble in excess red-brown ppt is Fe(OH)3. | red-brown ppt., insoluble in excess red-brown ppt is Fe(OH)3. |
| lead(II) (Pb2+) | white ppt., soluble in excess giving a colourless solution white ppt is Pb(OH)2. | white ppt., insoluble in excess white ppt is Pb(OH)2. |
| zinc (Zn2+) | white ppt., soluble in excess giving a colourless solution white ppt is Zn(OH)2. | white ppt., soluble in excess giving a colourless solution white ppt is Zn(OH)2. |
Some items to note for testing of cations:
- To distinguish between lead(II) ions and aluminium ions, add a solution containing chloride. Lead(II) ions will form a white precipitate of lead(II) chloride upon adding of the chloride solution. For the solution containing aluminium ions, no white precipitate will be observed.
- All the precipitates formed in the cation qualitative analysis table above are hydroxides (even when we add aqueous ammonia). The hydroxide ions that form the precipitate from the solution, is formed from the dissociation of ammonia in water (not in O Level Chemistry syllabus).
NH3 + H2O ⇌ NH4+ + OH–
How to remember the cation table easily?
I would group similar ones together.
- First, start off my remembering that we use aqueous sodium hydroxide and aqueous ammonia to test for the cations.
- Next, remember that there are 3 cations that formed coloured (i.e. non- white) precipitate.
copper (II) –> blue
iron (II) –> blue
iron (III) –> red-brown
- The rest form white precipitate, and give colourless solution (if the precipitate are soluble)
- Al3+, Pb2+ and Zn2+ forms precipitate soluble in excess aqueous sodium hydroxide
- Cu2+ and Zn2+ forms precipitate soluble in excess aqueous ammonia
- ammonia is different (it needs heating with NaOH, and we test for gas instead of observe precipitate)
- Ca2+ does not give a precipitate with aqueous ammonia
Qualitative Table for Anions
| anion | test | test result |
| carbonate (CO32-) | add dilute acid | effervescence, carbon dioxide produced |
| chloride (Cl–) | acidify with dilute nitric acid, then add aqueous silver nitrate | white ppt |
| iodide (I–) | acidify with dilute nitric acid, then add aqueous silver nitrate | yellow ppt. |
| nitrate (NO3–) | add aqueous sodium hydroxide, then aluminium foil; warm carefully | ammonia produced |
| sulfate (SO42-) | acidify with dilute nitric acid, then add aqueous barium nitrate | white ppt. |
Some items to note:
- The tests for chlorides, iodide and sulfate involve precipitation. Hence, both reagents should start off as aqueous solutions. Do also note that precipitation reaction will precipitate out ionic compounds that are insoluble in water (see solubility of ionic compounds for details).
- For the test for chlorides and iodides, aqueous lead(II) nitrate could be used instead of aqueous silver nitrate. chloride ions will give a white ppt with lead(II) nitrate, while iodide ions will give a yellow ppt with lead(II) nitrate.
- Take a look at the test for nitrates vs the test for ammonium ions (see cation table). You will notice that the difference lies in whether aluminium foil is used or not. Since test for ammonium ions does not require aluminium foil, this means that whether you use aluminium foil or not, if there is ammonium ions, ammonia gas will be produced. However, for nitrate, we need to use aluminium foil in order for ammonia to be generated upon heating.
Testing for gases
| Gas | Test and test result |
| ammonia (NH3) | turns damp red litmus paper blue |
| carbon dioxide (CO2) | gives white ppt. with limewater |
| chlorine (Cl2) | bleaches damp litmus paper |
| hydrogen (H2) | ‘pops’ with a lighted splint |
| oxygen (O2) | relights a glowing splint |
| sulfur dioxide (SO2) | turns aqueous acidified potassium manganate(VII) from purple to colourless |
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