A glowing splint relights in oxygen
- A wooden splint is lit and then blown out, so that it is glowing but not burning.
- The glowing splint is put into the mouth of the test tube.
- In oxygen the splint relights, bursting back into flame.
- The relighting is the positive result that identifies oxygen.
- A splint that simply goes out shows the gas is not oxygen.
Key Idea
The splint must be glowing, not burning, or the result cannot be read.
Why the test works
- A glowing splint is burning slowly, held back by the small amount of oxygen in air.
- Pure oxygen is about five times as concentrated as the oxygen in air.
- More oxygen means the wood burns fast enough to produce a flame again.
- Combustion needs oxygen, so the gas is taking part in the reaction rather than just being present.
- The test is therefore a chemical test, not simply an observation.
Note
Air itself will not relight a glowing splint, which is what makes the result meaningful.
Carrying out the test and reading it
- The gas is collected so that it does not escape before the test is made.
- The splint is lowered in promptly, because the gas mixes with air over time.
- A clear relight is the only result that confirms oxygen.
- A splint that stays glowing but does not flame is not a positive result.
- Naming the gas and the observation is what makes an answer complete.
Common Mistake
Relighting the splint is specific to oxygen, and should not be confused with the squeaky pop that shows hydrogen.
Self review
- Describe the chemical test for oxygen.
- What state must the splint be in before it is used?
- What is the positive result?
- Why does a glowing splint relight in oxygen but not in air?
- Which gas gives a squeaky pop instead?
