What is the density of liquid air at its highest temperature? - Sue Kamal (sakamal@bentley.unco.edu)

I can give you a quick back-of-the-envelope estimate, but I don't have that data.
Cryogenic properties of gases, taken from the CRC Handbook of Chemistry and Physics
GasDensity, kg/m3Boiling point at 1 atm, K
O2114290.18
N280477.35
Ar139087.45
The highest temperature for liquid air is the boiling point for nitrogen gas. If I assume that liquid air is an "ideal solution" (that is, all the molecules in the mixture interact the same way, whether they're oxygen, nitrogen, or argon), I can write

density of liquid air = 0.78084 × density of nitrogen + 0.20946 × density of oxygen + 0.00934 × density of argon

where the fractions are the volume fractions of each component in pure, dry air. (They don't add up to 1 because I'm ignoring all the trace gases, including CO2, to keep things simple). If I further assume that the densities of liquid oxygen and liquid argon aren't much different at the boiling point of nitrogen then they are at their own boiling points, I can write the density of liquid air at the boiling point of nitrogen as
(0.78084 × 804 kg/m3) + (0.20946 × 1142 kg/m3) + (0.00934 × 1390 kg/m3) = 880 kg/m3
or a bit less dense than water is around room temperature.

What's the difference between an alloy and an ore?

Antoine answers:

An alloy is a homogeneous mixture of elements; at least one of the elements must be a metal. An ore is a naturally occuring mixture (usually heterogenous) that contains a particular metal (or a compound of that metal) in amounts that make extraction of the metal economically worthwhile.

For more about mixtures, see General Chemistry Online's section on classification of matter.

I want to know the easiest way to remove sodium chloride from a solution and still retain the other minerals.

sireno@netonecom.net

Antoine answers:

Dissolved salts are usually removed using either deionization or reverse osmosis. Deionization passes the water over ion exchange resins, which adsorb dissolved ions. Reverse osmosis is what Kevin Costner did in the opening scene of "Waterworld": force pressurized water across a membrane which is impermeable to dissolved salts. Neither is very easy, in practice. And neither will selectively remove sodium chloride.

The simplest approach would probably be deionization, followed by replacement of the dissolved minerals removed in the deionization.

  1. Determine the concentration of the mineral components you want to keep.
  2. Pass the water through a deionizer. This is a column packed with mixed cation and anion exchange resins. The cation exchange resin will remove the Na+ ions, replacing them with H+ ions. It will also replace the calcium and magnesium and other cations. The anion exchange resin will replace the chloride with hydroxide ions. The hydroxide ions will neutralize the hydrogen ions produced in the previous step. The anion exchange resin will also take sulfate out of the water.
  3. Replace the mineral components lost in the cation and anion removal processes.

The alternative approach, reverse osmosis, is outlined here.