A student collects a gas produced during an experiment and pushes a glowing (but not flaming) wooden splint into the mouth of the test tube. The splint suddenly bursts back into flame. What does this confirm about the gas, and why does the splint relight?
- The gas is hydrogen, because hydrogen is highly flammable and ignites the moment it touches a hot splint
- The gas is oxygen, because oxygen supports combustion, so it provides enough of what the glowing splint's embers need to burst back into flame
- The gas is carbon dioxide, because carbon dioxide fuels burning in exactly the same way oxygen does
- The gas is water vapour, which condenses on the hot splint and produces a small spark from the sudden temperature change
Why B? And why not the others?
Correct answer: B. The gas is oxygen, because oxygen supports combustion, so it provides enough of what the glowing splint's embers need to burst back into flame
A glowing splint relighting is the standard school test for oxygen: oxygen supports combustion, so surrounding the glowing embers with a higher concentration of it than normal air provides exactly what's needed for the smouldering wood to burst back into an open flame. The option naming hydrogen is wrong because hydrogen is tested for differently - it produces a distinctive squeaky pop when a lit (flaming) splint is held near it, igniting the gas itself, rather than relighting a glowing ember. The option naming carbon dioxide is wrong because carbon dioxide does the opposite of supporting combustion: it is commonly used to put fires out, and a glowing splint placed in carbon dioxide goes out completely rather than relighting. The option naming water vapour is wrong because water vapour has no chemical property that would cause a splint to reignite, and condensation on a hot splint would, if anything, tend to cool and dampen it rather than produce a flame.
Source: BBC Bitesize GCSE Chemistry: Tests for gases -- oxygen (glowing splint test)