When a bumine spinter is broughi near the mouth of a test tube contening ces X i glows brighiy. Gas X is
Answers
Safety: Eye protection must be used for any chemical testing, especially burning hydrogen which is a small, (hopefully) controlled explosion.
To test gases, you will get a better result by using boiling tubes rather than test tubes. Ideally thick walled boiling tubes are best for the hydrogen test since they are less likely to break.
Oxygen: The burning splint will burn much more brightly once inserted into the oxygen gas. If a wooden spill is used then you may notice that the flame burns with a slight pinky / lilac colour. This is due to the presence of potassium in wood. (wood ash boiled with animal or vegetable fats is how the firts soaps were produced, since fresh wood ash contains potassium hydroxide.)
Hydrogen: This test can often fail due to not appreciating the process and how hydrogen behaves. Wheareas oxygen has a density roughly the same as air, hydrogen is much less dense. For this reason the tube should be held upside down until ready to use it - ideally unstoppered in a beaker with a small amount of water at the bottom to form a seal (stoppers can be used but the time taken to stopper and then unstopper the tube gives the hydrogen opportunity to escape). The test should be completed as quickly as possible, again, to minimise the loss of hydrogen…
Hold the tube upside down, ideally with the opening under water where the gas was collected by water displacement.
Take the tube to the flame, not the other way round.
I prefer to use a yellow bunsen flame for this test, but a splint will do. Hold the splint steady and in a single action, lift the boiling tube and turn the tube (perhaps to an angle of about 45 degrees) such that the opening is just below the flame. Hydrogen will float out of the tube and into the flame. With practice this action takes a small fraction of a second.
Wait maybe a couple of seconds at most. Hydrogen doesn’t necessarily give the “squeaky-pop test” since it is not actually a feature of burning hydrogen. As the hydrogen floats out of the tube it is replaced by air. Once there is an “explosive mixture” of hydrogen and oxygen in the tube - THAT’S when the pop happens. The flame shoots into the tube and rapidly burns down making the “column” of hydrogen shorter, the sound made is a feature of the length of the hydrogen column and the pitch increases as the column gets shorter. If you don’t get the explosive mixture, then the pop test fails - it’s the mixture that is critical, not the hydrogen.
SAFETY: Do not, under any circumstances, mix hydrogen and oxygen in a glass container of any sort - especially if you plan to hold it, as the glass vessel is likely to shatter sending glass shards into your face and hands - not good. Nitrogen in the air acts as a moderator to reduce the effect of the exploding (rapidly burning) hydrogen. Eye protection must always be worn when testing for hydrogen.
An interesting extension to the pop-test (for teachers)…
This requires boiling tubes that are absolutely dry inside. Dry boiling tubes can be obtained by warming boiling tubes in a medium bunsen flame until they are warm to the touch - not hot - or use a drying oven if you have one. This removes the layer of water molecules that are always adsorbed onto the surface of glass. Stopper while warm until ready to collect hydrogen. Thick walled tubes are best since they are less likely to break and stay cold for longer.
Collect hydrogen by upward displacement. Hydrogen is best obtained from a cylinder if you have one since the gas is already dry, otherwise prepare it in a Kipp’s apparatus and pass the gas through conc. sulfuric acid to dry the hydrogen - not for a class to do! Stopper the tube until ready to test.
Place the stoppered, dry hydrogen-filled tubes in a refrigerator to cool.
Light a bunsen in a fume cupboard and use a yellow flame to burn the hydrogen in the test.
Have a paper towel ready to remove condensation that will probably collect on the outer surface of the tube.
Wear eye protection.
In a smooth action: take a tube (ideally in a wooden holder), remove the stopper, place the tube near the base of the bunsen flame holding the tube at about 45 a degree angle so that hydrogen floats out and into the flame.
Once you get the squeaky pop, make sure there is no condensation outside the tube and look closely for condensation inside the tube. If you are lucky you will see concentric bands of condensation spaced a few cm apart. Try viewing against a dark background. You may need to practice this process
For this to work it is essential that the glass remains as cold as possible (hence the cooling) is not warmed up by heat from the hands (hence wooden holder) or the flame (hence need to work quickly). If the tube and gas were prepared dry, then the condensation inside the tube must be due to combustion of hydrogen. The spacing of rings is due to a standing wave being set up inside the tube which gives the squeaky pop.
Any condensation observed can be tested with a small piece of blue cobalt chloride paper.