Which one of the following gases has the highest solubility in water?

Correct — B, (b) Ammonia. Ammonia is not merely the most soluble of these four gases; it is one of the most soluble gases known, dissolving to the extent of about 530 grams in a litre of water at 20 degrees Celsius. Set that beside carbon dioxide, which manages about 1.45 grams per litre at 25 degrees and near atmospheric pressure, and the gap is not a matter of degree — it is a factor of several hundred.

The reason is hydrogen bonding. Ammonia is a polar molecule with a lone pair on the nitrogen and three hydrogen atoms bonded to a highly electronegative atom, which is exactly the arrangement that lets it hydrogen-bond to water in both directions: the nitrogen lone pair accepts a hydrogen bond from a water molecule, and the ammonia hydrogens donate hydrogen bonds to water oxygens. Water is itself a hydrogen-bonded liquid, so an ammonia molecule slips into that network rather than having to displace it. A small part of the dissolved ammonia goes further and reacts, taking a proton from water to give ammonium and hydroxide ions, which is why aqueous ammonia is alkaline and turns red litmus blue.

The classic demonstration is the fountain experiment. A dry flask filled with ammonia is fitted with a jet dipping into water; a few drops admitted into the flask dissolve so much of the gas that the pressure inside collapses, and the water is driven up the jet as a fountain, coloured pink by an indicator because the solution is alkaline. Only a gas of extraordinary solubility can produce that effect, and hydrogen chloride, the other gas used for the same demonstration, is the other member of that small class.

The general rule this item is testing is the old one that like dissolves like, refined for gases: a gas dissolves well in water if it is polar, if it can hydrogen-bond, or if it reacts with water. Ammonia does all three. Nitrogen does none of them, and it is the least soluble of the four by a wide margin.

Why the others are wrong

• (a)Chlorine — Chlorine is a great deal more soluble in water than nitrogen but nowhere near as soluble as ammonia. The molecule is non-polar, so it cannot hydrogen-bond to water; what solubility it has comes largely from a reaction rather than from simple dissolution, chlorine disproportionating in water to give hypochlorous acid together with hydrogen and chloride ions, and even that reaction sits at an equilibrium well over to the left. The resulting solution is the chlorine water of the laboratory and, in far greater dilution, the basis of water disinfection. Chlorine is a reasonable second guess on this item, but it is a second guess.
• (c)Carbon dioxide — Carbon dioxide is only slightly soluble — about 1.45 grams per litre at 25 degrees Celsius and near atmospheric pressure. It is a linear molecule with two equal and opposite bond dipoles that cancel, so it is non-polar overall and cannot hydrogen-bond to water; and although it does react with water to give carbonic acid, the equilibrium is far to the left and the great majority of the dissolved gas stays as carbon dioxide molecules. This option is chosen more often than any other wrong answer, because a fizzy drink looks like a lot of dissolved gas. It is not: the gas in a bottle is held there by pressure, and Henry’s law is what puts it in — increase the pressure and more dissolves, release the pressure and it comes back out, which is the hiss when the cap is lifted.
• (d)Nitrogen — Nitrogen is by a wide margin the least soluble of the four. The molecule is non-polar, held together by a triple bond that makes it chemically inert at ordinary temperatures, and it neither hydrogen-bonds to water nor reacts with it, so nothing pulls it into solution but weak dispersion forces. This is why nitrogen makes up about four-fifths of the atmosphere yet contributes so little to the dissolved gas content of natural water, and it is also why nitrogen is the gas responsible for decompression sickness in divers: what little dissolves in the blood under pressure at depth comes out as bubbles if the pressure is released too quickly.

The solubility of a gas in water is governed by two things. The first is the strength of the interaction between the gas molecule and water. Non-polar gases interact with water only through weak dispersion forces and are sparingly soluble; polar gases do better; gases that can hydrogen-bond to water do better still; and gases that react with water — forming ions or a new species — do best of all, because the reaction removes dissolved molecules from solution and pulls more gas in after them. Ammonia and hydrogen chloride sit at the top of that scale, both of them so soluble that a fountain can be raised with them. Carbon dioxide and sulphur dioxide are moderately soluble, chlorine somewhat similar, and hydrogen, nitrogen and the noble gases are at the bottom. The second governing factor is physical conditions. Henry’s law states that the amount of a gas dissolved in a liquid at a given temperature is proportional to the partial pressure of that gas above the liquid, which is why a carbonated drink is bottled under pressure and effervesces when opened. And unlike most solids, gases become less soluble in water as the temperature rises, because dissolution of a gas releases energy and warming drives the equilibrium back — which is why boiled water tastes flat, why warm river water holds less dissolved oxygen, and why thermal pollution of a river harms the fish in it.

Key facts

• Ammonia is among the most soluble of all gases in water, dissolving to about 530 grams per litre at 20 degrees Celsius.
• Ammonia is polar and hydrogen-bonds to water both as donor and as acceptor; a small fraction also reacts, giving ammonium and hydroxide ions, so the solution is alkaline.
• Carbon dioxide dissolves to about 1.45 grams per litre at 25 degrees Celsius and near atmospheric pressure, and most of it remains as carbon dioxide molecules rather than carbonic acid.
• Chlorine is non-polar and dissolves partly by disproportionating in water to hypochlorous acid, hydrogen ions and chloride ions.
• Nitrogen is non-polar and inert, and is the least soluble of the four.
• Henry’s law: at a given temperature, the mass of a gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid.
• The solubility of gases in water decreases as temperature increases, the opposite of the usual behaviour of solids.
• The ammonia fountain experiment demonstrates the gas’s solubility: a few drops of water in a flask of ammonia collapse the pressure and drive a fountain of water up into it.

Common traps

• Assuming carbon dioxide must be highly soluble because fizzy drinks contain so much of it. It is held in by pressure, not by solubility.
• Reasoning from the abundance of a gas in the atmosphere to its solubility. Nitrogen is the most abundant and the least soluble of the four.
• Forgetting that gas solubility falls as temperature rises, unlike that of most solids.
• Treating chlorine as highly soluble because chlorine water is a familiar reagent. Its dissolution depends on a reaction that lies well to the left.

• practice — not a real PYQ

  The solubility of a gas in a liquid at a given temperature is directly proportional to the partial pressure of the gas above the liquid. This statement is

  • (a)Raoult’s law
  • (b)Henry’s law
  • (c)Dalton’s law
  • (d)Graham’s law

  Answer(b) Henry’s law

• practice — not a real PYQ

  An aqueous solution of ammonia is

  • (a)strongly acidic
  • (b)weakly acidic
  • (c)neutral
  • (d)alkaline

  Answer(d) alkaline