Solubility

Ammonia is a polar solvent that can form hydrogen bonds. Water, has exactly the same characteristics so it is the most soluble in ammonia. CH₂Cl₂ is polar but cannot form hydrogen bonds so it is not as soluble in ammonia as is water. CH₄ is completely non-polar and is, therefore, the least soluble in ammonia. The correct order is:

H₂O, CH₂Cl₂, CH₄

a. Water is better because ethanol is polar and can form hydrogen bonds. b. Hexane is better because CCl₄ is a non-polar molecule. c. Water is better because H₂S is polar. d. Hexane is better because benzene is non-polar.

When you siphon out the water, you create a larger volume for the nitrogen gas to fill which causes a decrease in nitrogen pressure. By Henry's law, that will make the concentration of nitrogen dissolved in the water decrease.