Vapour Density
9 Minutes
Vapour density for students: learn how vapor differs from gas, how relative vapor density is calculated, and why gases sink. A science lesson in English.
- Differences between pure gas and vapor using water and steam
- Measuring relative vapor density using hydrogen as a reference
- Why dense gases like carbon dioxide and chlorine sink in air
What this lesson teaches
Vapour density is the density of a vapour or gas measured relative to a standard reference, such as hydrogen gas or atmospheric air. It describes how heavy a given volume of gas is under fixed conditions of temperature and pressure.
Difference between gas and vapour
- A pure gas consists of atoms or molecules spread so far apart that they cannot be seen.
- A vapour is not in a pure gaseous state, but is a mixture of a gas combined with liquid or solid particles.
- Invisible steam is pure gaseous water, whereas the visible cloud rising from boiling water is water vapour containing liquid droplets.
- Heating water vapour completely converts those suspended liquid droplets into invisible steam.
Effects of temperature and pressure
Unlike solids and liquids, gases and vapours have no fixed volume. When the temperature drops, such as when a balloon is placed in liquid nitrogen, the particles move closer together and the volume shrinks. When temperature increases, particles move more freely and volume expands. Increasing the pressure inside a container, like pushing the plunger of a sealed syringe, also compresses the gas and reduces its volume.
Measuring relative vapour density
- Density is tested at standard conditions of 0 degrees Celsius and atmospheric pressure.
- Hydrogen is used as the reference because hydrogen has the lowest atomic mass of any element.
- Relative vapour density equals the molar mass of a gas divided by the molar mass of a hydrogen molecule, which is two times the mass of a hydrogen atom.
- This is also equal to the mass of one molecule of the gas divided by two times the mass of one hydrogen atom.
- For oxygen, sixteen moles of hydrogen atoms are needed to balance one mole of oxygen, giving oxygen a relative vapour density of sixteen.
Why dense gases sink
For practical everyday comparisons, atmospheric air is used as the reference instead of hydrogen. When the ratio of a gas's molar mass to that of air is greater than one, the gas is denser than air and settles downward. For instance, carbon dioxide vapour from dry ice stays low and crawls along the ground. During the Second Battle of Ypres, dense chlorine vapour sank into French trenches for this same reason.