|
|
 Originally Posted by OngBonga
Ok so let me see if I understand why humid air is less dense than dry air...
Dry air is mostly made up of oxygen and nitrogen. Water is oxygen and hydrogen. Hydrogen is much lighter than nitrogen. Furthermore, the oxygen in the atmosphere is O2 (nitrogen is N2 also, I believe), but there is only one atom of oxygen in a water molecule. Thus, dry air is significantly heavier than humid air as it contains more oxygen, and nitrogen.
It just seems crazy to think that water is lighter than air on a molecular level. But it's also makes sense when you look at water's structure. Two tiny hydrogen atoms and one oxygen atom. That's ten electrons per molecule. That's less than both N2 and O2, before we consider CO2 and the other trace gasses.
It's no good to count electrons for the mass. A proton has about the same mass as a neutron, and both are about 1,800 times as massive as an electron. Furthermore, counting electrons is equivalent to counting protons (for atoms, not ions), but ignores the neutrons.
***
The point that you left out is Avogadro's Law. Avogadro hypothesized that equal volumes of gas will contain equal number of particles if they are at the same temperature and pressure.
Essentially, this says that the volume any gaseous molecule "occupies" is the same, regardless of its atomic constituents. It doesn't mean that the molecules are the same volume. A lighter particle with the same kinetic energy as a heavier particle will move more quickly. It covers more distance between particle collisions, and occupies a greater volume as such.
***
Using Avogadro's Law and further simplifying out the mass of the electrons and assuming all generations of atom are of the most common isotope, and that E=mc2 isn't acting to increase the mass of the nucleus due to bonding energy:
(All of these are small effects, compared to the result we will get.)
The air is mostly N2 and O2, which have equal numbers of neutrons as protons in their nuclei. This fact is not obvious, and has been obtained by observation. It was later shown via QM that this is a predictable result.
Nitrogen atoms have 7 protons and 7 neutrons, giving an atomic mass of 14. The molecule N2 has atomic mas of 28.
Oxygen atoms have 8 protons and 8 neutrons, giving an atomic mass of 16. The molecule O2 has atomic mas of 32.
Hydrogen atoms have 1 proton and 0 neutrons, giving an atomic mass of 1.
Water atoms have 1 Oxygen and 2 Hydrogens, giving an atomic mass of ( 16 + 1 + 1 = ) 18.
***
Taking the assumption that all 3 molecules occupy the same volume, water is the least dense.
EDIT: water atoms? lol. water molecules, obv.
|
Reply With Quote