杏吧原创

Free the atoms

Oxygen has a slightly greater density than nitrogen. Why don鈥檛 these main constituents of air separate out?

鈥� Gas molecules move rapidly at room temperature, with oxygen and nitrogen travelling at around 500 metres per second, so they obviously collide frequently. This allows the oxygen and nitrogen molecules to mingle and mix, rather like large numbers of people on a nightclub dance floor, in a process known as diffusion. Convection, the transfer of heat within the atmosphere,also plays an important role in gas mixing.

Gas mixing is a spontaneous process. This means that if you had a container with two compartments separated by a barrier, with one compartment containing pure nitrogen and the other pure oxygen, the two gases would automatically mix as soon as the barrier was removed.

Kenneth Koon

By email, no address supplied

鈥� A change in the ratio of oxygen to nitrogen would be expected in a hypothetical quies鈥檆ent atmosphere. However, constant mixing occurs in the real atmosphere, driven by the Earth鈥檚 rotation and also by differences in density between hot air at the Earth鈥檚 surface and colder air higher up.

Up to altitudes of between 80 and 120 kilometres this mixing results in a fairly uniform concentration of oxygen and nitrogen 鈥� which respectively make up approximately 21 per cent and 78 percent of the atmosphere.

This region is known as the homosphere. Partial stratification of the two gases does occur above 120 kilometres, in the heterosphere, where the density of air is much lower than at the surface and the efficiency of bulk mixing processes is reduced.

Simon Iveson and Mark Hentschel

Alumni of University of Newcastle, New South Wales, Australia

鈥� If there were no circulation in the atmosphere, the oxygen would tend to concentrate in the lower strata. This process would take millions of years once circulation ceased because molecules of oxygen (and, indeed, nitrogen) are constantly colliding with other molecules, meaning that it would take a long time for a particular molecule to fall from its starting point to the ground. Once it hit the ground, it would bounce and eventually rise again to a great height, only to fall again. This would be repeated frequently if no other variable, such as temperature, changed.

Although the individual molecules continue to travel up and down, each 鈥渟pecies鈥� of oxygen and nitrogen would eventually reach an equilibrium distribution of molecules per unit volume as a function of height. This species density will decrease with height by an amount that depends on the molecular weight of the species. So the oxygen would fall off with height slightly faster than the nitrogen. At the surface, this would give a slightly higher concentration of oxygen than we experience now. At high altitudes, the air would become richer in nitrogen, but then other gases such as water vapour, neon, methane, helium and hydrogen would dominate.

In fact, atmospheric circulation and turbulence prevents this from happening in the lower atmosphere. But in the very high atmosphere there is not much circulation and the composition does indeed become dominated by atomic oxygen. Above 600 kilometres this is superseded by helium, and eventually by atomic hydrogen.

Eric Kvaalen

Paris, France

鈥� The average speed of an oxygen molecule at 27 掳C is calculated at 484 metres per second. If you mix in slightly lighter nitrogen molecules those will move even faster. Even at those speeds, the molecules will collide with each other after travelling only about 68 nanometres. This gives the molecules no chance to settle.

It is like entering a classroom full of teenage boys and girls at the height of puberty on the last day before the summer break and patiently expecting them to settle down to serious work. It won鈥檛 happen.

Sjoerd Spoelstra

Physics teacher

Rotterdam, Netherlands

This week鈥檚 questions

Complex colouring

I was taught at university that brown eyes were genetically dominant to blue eyes, and that two blue-eyed parents could not have a brown-eyed child, only one with blue eyes. I now understand that recent research has proved this to be untrue and that in some circumstances two blue-eyed parents can have a brown-eyed child. What has the new research uncovered and how does the newly discovered genetics work?

Alan Wheal

Birkenshaw, West Yorkshire, UK

Foaming at the trunk

This morning, while walking my dog through an old plantation of Monterey pines (Pinus radiata), I noticed that the trees were frothing. The froth looked like soapsuds dripping out of cracks in the bark. Is this a chemical reaction, or a result of biological action? It was an exceptionally cold first month of winter, and the previous day it had been raining continuously until early afternoon, when it turned out clear and sunny. However, more overnight rain was still falling when the photographs were taken. The foam was only evident on the wetted parts of the bark, and not on dead or older, mature trees.

Jan Horton

West Launceston, Tasmania, Australia