collecting gases technique

[Scafalon40]

lets just say that in a paper 3 exam we carry out a test on an aqueous solution of ions(salt analysis)
the question paper asks us to write down any observations
how will we know if a gas is produced or not?
i mean if its something like CO2 being produced, ill notice the fizzing; but what about ammonia?
and must i pass CO2 through limewater to verify the result or is writing down "fizzing" enough?
and if the gas is not CO2, do i need to carry out all the other tests for gases to find out what it is( if i notice a gas being produced in the first place!) That would take a lot of time, and i would only have a limited sample of the gas; i can't carry out the same test for the ion in a test tube over and over again!
please help

 

[Xenon]

lets just say that in a paper 3 exam we carry out a test on an aqueous solution of ions(salt analysis)
the question paper asks us to write down any observations
how will we know if a gas is produced or not?
i mean if its something like CO2 being produced, ill notice the fizzing; but what about ammonia?
and must i pass CO2 through limewater to verify the result or is writing down "fizzing" enough?
and if the gas is not CO2, do i need to carry out all the other tests for gases to find out what it is( if i notice a gas being produced in the first place!) That would take a lot of time, and i would only have a limited sample of the gas; i can't carry out the same test for the ion in a test tube over and over again!
please help

mainly these are the gases along with their identification method that you'll be required to perform during exam

ammonia>> NH3 turns damp red litmus paper blue
carbon dioxide>> CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2)
chlorine>> Cl 2 bleaches damp litmus paper
hydrogen>> H2 ‘pops’ with a lighted splint
oxygen>> O2 relights a glowing splint
sulfur dioxide>> SO2 turns acidified aqueous potassium dichromate(VI) from orange to green
another one that might help you is that of NO2 is the (pale) brown colour of NO2 in air

as far as I remember, the questions do not directly ask to identify a gas, you might be asked to verify the presence of say nitrate, NO3–(aq) ions...in such cases you do know that NH3 liberated on heating with OH–(aq) and Al foil, so only test for ammonia... if litmus paper changes colour than yes NO3- is present.
So you'll get hint from the question and also the apparatus and chemicals you'll be provided, say again that if you given splints you can guess that you might be required to perform H2 or O2 test. And also sometimes, you can detect the gases from just observation, fizzing indicate CO2, a brown gas indicates NO2....
and with further practice you'll eventually get a better handle on this .. no need to worry

 

[Scafalon40]

mainly these are the gases along with their identification method that you'll be required to perform during exam

ammonia>> NH3 turns damp red litmus paper blue
carbon dioxide>> CO2 gives a white ppt. with limewater (ppt. dissolves with excess CO2)
chlorine>> Cl 2 bleaches damp litmus paper
hydrogen>> H2 ‘pops’ with a lighted splint
oxygen>> O2 relights a glowing splint
sulfur dioxide>> SO2 turns acidified aqueous potassium dichromate(VI) from orange to green
another one that might help you is that of NO2 is the (pale) brown colour of NO2 in air

as far as I remember, the questions do not directly ask to identify a gas, you might be asked to verify the presence of say nitrate, NO3–(aq) ions...in such cases you do know that NH3 liberated on heating with OH–(aq) and Al foil, so only test for ammonia... if litmus paper changes colour than yes NO3- is present.
So you'll get hint from the question and also the apparatus and chemicals you'll be provided, say again that if you given splints you can guess that you might be required to perform H2 or O2 test. And also sometimes, you can detect the gases from just observation, fizzing indicate CO2, a brown gas indicates NO2....
and with further practice you'll eventually get a better handle on this .. no need to worry

Thanks,
Funny thing is that a guy whose username is technically a gas, replied to my post

 

[MindStealth]

When you write the observation for some gas that is released, always include a confirmatory test. You should actually never perform these confirmatory tests (to save time) just use your knowledge of chemistry to find which gas is released but always include the test in the observations. E.g. when a carbonate is reacted with an acid, you should write the observation; "Colorless gas released which turns lime water milky." The examiners don't expect you to test the gases so you will not be given any gas which a normal student is not familiar with. 80% of the time its ammonia, the test is simple just smell it. The pungent smell of ammonia is easily distinguishable. Just make sure you have it memorize though, by that I mean you should have smelled it enough times to know what it actually is. It cannot be explained in words.

For more tips on Chemistry P3, refer : http://www.xtremepapers.com/community/threads/tips-for-chemistry-practicals-p3.11810/

 

[VicBoss]

S07 p31 colourless gas is released (effervescent observed ) by the addition of acid. So it can be CO2 or SO2 (its co2) ,how am i suppose to know which one it is without carrying out a test or using least time possible?