Test for cations using aqueous sodium hydroxide/aqueous ammonia

Read the list of instructions before you view the videos.
Pay attention to the WAY the test is done.

If you use the correct volumes and do the tests in the correct way, you will get the correct observation.

Instructions for doing cation tests:
1. Put about 2cm3 of unknown into a test tube.
2. Do NOT use more than 2cm3.
3. Add A FEW DROPS of aqueous sodium hydroxide/ammonia.
4. Shake the test tube to mix.
5. Observe for ppt.
6. Add aqueous sodium hydroxide/ammonia until it's 3/4 of the test tube. (i.e. excess).
7. Observe whether ppt is soluble in excess or not.


1) Ammonium ion test

Observation: No ppt formed. Ammonia gas is produced on warming. (refer to 'ammonia gas test' video)



2) Calcium ion test

Observation: White ppt formed in aq. sodium hydroxide, insoluble in excess. No visible change in aq. ammonia.



3) Zinc ion test

Observation: White ppt formed in aq. sodium hydroxide, soluble in excess to form colourless solution.
White ppt formed in aq. ammonia, soluble in excess to form colourless solution.



4) Aluminium/Lead ion test

Observation: White ppt. formed in aq. sodium hydroxide, soluble in excess to form colourless solution. White ppt formed in aq. ammonia, insoluble in excess.






5) Iron (II) ion test

Observation: Green ppt formed in aq. sodium hydroxide, insoluble in excess. Green ppt formed in aq. ammonia, insoluble in excess.



6) Iron (III) ion test

Observation: Reddish brown ppt formed in aq. sodium hydroxide, insoluble in excess. Reddish brown ppt formed in aq. ammonia, insoluble in excess.



7) Copper (II) ion test

Observation: Blue ppt formed in aq. sodium hydroxide, insoluble in excess. Blue ppt formed in aq. ammonia, soluble in excess to form dark blue solution.

White ppt vs No ppt formed (solution)

Ppt = SOLID is present



1. Test tube 1 contains a colourless SOLUTION.
2. Test tubes 2, 3 and 4 ALL contain WHITE PPT.
3. There is NO SUCH THING as a "white solution" or a "colourless ppt".

Observations for this experiment:
Test tube 1: Colourless solution formed.
Test tubes 2/3/4: White ppt formed.


SOLUTION = no solid present


1. All these test tubes contain SOLUTIONS.
2. You can tell a test tube contains a solution because there is NO SOLID in it at all.
3. If you are not sure, hold the test tube against white light (e.g. window) or a white piece of paper to see clearer.
4. If after mixing, you get any of the above results, you will describe your observations as:

Test tube 1 = Dark blue solution formed
Test tube 2 = Green solution formed
Test tube 3 = Purple solution formed
Test tube 4 = Orange solution formed
Test tube 5 = Yellow solution formed
Test tube 6 = Pink solution formed
Test tube 7 = Pale blue solution formed

(courtesy of Ms Bernice Chen)

2cm3

1. 2cm3 of solution is about 1.5cm3 in height in a test tube.
2. A rough estimate of the volume is good enough.
3. Use your little finger to estimate.
4. Do NOT waste time measuring exactly 2cm3 of solutions.


(courtesy of Ms Bernice Chen)

Test for Gases

1) HYDROGEN GAS - colourless and odourless



Test for gas: hold a lighted splint over the mouth of the test tube.
Observation: Lighted splint extinguished with a 'pop' sound.

2) OXYGEN GAS - colourless and odourless




Test for gas: Place a glowing splint into the mouth of the test-tube.
Observation: Glowing splint is rekindled.

3) CARBON DIOXIDE GAS - colourless and odourless

refer to 'test for carbonate ion' video

Test for gas: Pass the gas into calcium hydroxide(limewater) solution.
Observation: White ppt formed in limewater.
(Limewater turned chalky/milky is NOT accepted.)

4) CHLORINE GAS - yellowish green and choking


Test for gas: Hold a damp blue litmus paper over the mouth of the test-tube.
Observation: Blue litmus paper turns red and then bleached.

5) AMMONIA GAS - colourless and pungent


Test for gas: Hold a damp red litmus paper over the mouth of the test-tube.
Observation: Red litmus paper turns blue.

6) SULFUR DIOXIDE GAS - colourless and choking


Test for gas: Bubble the gas evolved into the potassium dichromate (VI) solution
Observation: Potassium dichromate(VI) solution turns from orange to green.