Students are unable to choose the appropriate test for identifying the gas evolved.
Typical Scenario:
When a gas evolved from a reaction, pupils used All the various test to identify the unknown gas.
Before the correct test can be used to test for the unknown gas, the reaction stopped to produce any gas!!!!
So how?!?
You can deduce the identity of a gas ( before actually testing ) by:
*** 1.The reactions that produced the gas.
2.The odour of the gas.
3.The colour of the gas, (whereby in your syllabus), chlorine is yellowish green.
For hydrogen gas
the reagents used will surely be acids and metal strips...
if you did not used any shiny powder ( probably metals ) or shiny strips, then the test for hydrogen is NOT NECESSARY because hydrogen gas will never be produce when the reagents are not acids and metals!!! so, please do not test with burning/lighted splints.
For carbon dioxide gas
carbon dioxide only evolved when you heat a carbonate or you react an acid with a carbonate. So, if you are not using a dull powder (or dull solids) then no need to test for carbon dioxide gas!!
For oxygen gas
oxygen gas will not be produced from acid-base/acids-metal/acids-carbonate reactions!!!!
oxygen gas reactions can be very tricky....
because there are a few reactions that produce oxygen gas.
Eg- adding manganese oxide to hydrogen peroxide and heating nitrates.
Pupils always expect the glowing splint to rekindle BRIGHTLY... however, in most reactions, this does not happen.
The reason is simple.... the amount of oxygen gas produced from your 1-heap spatula and 2cm3 of solvent is not enough to produce alot of oxygen gas!!!! Should you observe a slight glow/brightens, you can safely deduce the unknown gas as oxygen.
Lastly pupils need to know HOW TO CONCENTRATE GASES
Pupils need to use their thumb to cover the openng of the test-tube to 'concentrate' the gas.
This is to ensure the gases will be enough to create a clearer result. Most of the time, pupils failed to observe the result because the gas that travels from the reacting mixture to the opening of the test-tube is too little.
hope that you find this useful!
