Hydrogen, oxygen and nitrogen.

Step 1 of 5

The splint test

Hydrogen, oxygen and nitrogen are colourless, odourless gases, and in a closed jar you cannot tell them apart by looking. In the lab they are told apart by what they do to a flame. In the Scene a burning splint is brought to the mouth of a jar full of air, and it keeps burning quietly.

Step 2 of 5

The pop

Hydrogen is a flammable gas: it burns in oxygen. When the splint reaches the mouth of the jar, the hydrogen escaping from it mixes with the air and catches all at once, with a small pop. That is the test for hydrogen. Look at what is left on the glass after the pop.

Step 3 of 5

Nitrogen

In nitrogen the splint goes out. Nitrogen does not burn and does not support burning, because its molecule, $\mathrm{N_2}$⁠, holds its two atoms in a very strong triple bond. It is the gas that makes up about 78% of the air, which is why wood burns gently in air: only about a fifth of the molecules that hit it are oxygen.

Change Gas in the jar between the four options, and compare.

Step 4 of 5

Inside the jar

Zoom to the molecules is at Molecules: the jar has been closed, with hydrogen molecules inside. In a gas the average kinetic energy of a molecule depends only on the temperature, $\tfrac32 kT$⁠. From that you can work out the average speed:

$\bar v = \sqrt{\frac{8RT}{\pi M}}$$\bar v =$$\sqrt{\frac{8RT}{\pi M}}$⁠

where $M$⁠ is the molar mass in kilograms per mole.

Step 5 of 5

Light and fast

At the same temperature every gas has the same average kinetic energy, $\tfrac12 m v^2$⁠. So a light molecule moves faster: the speed goes inversely as the square root of the mass, $\bar v \propto \dfrac{1}{\sqrt M}$⁠. The jar now holds oxygen; compare its speed in the table beside the Scene with hydrogen’s.