Properties of carbon dioxide
The properties of carbon dioxide differ from those of both hydrogen and oxygen in one telling respect.
| Property | Why it matters |
|---|---|
| Denser than ordinary air | can be collected by upward displacement of air |
| Relative molecular mass is 44 | greater than the average value for air |
| Neither burns nor supports combustion | can be used in fire extinguishers |
| Slightly soluble in water | downward displacement of water also works |
| Colourless; has no smell | a test is needed |
Combustion across the three gases
Hydrogen is combustible. Oxygen supports combustion. Carbon dioxide is neither — it does not support combustion, and that is exactly why it has its main use.
Density opens a second method
CO2 is only slightly soluble in water, so downward displacement of water works; and being heavier than air, it can also be collected by upward displacement of air — the reverse of the method used for hydrogen.