Which one of the following gas is used in making soft drinks?

Correct — D, CO₂. A soft drink is water with carbon dioxide forced into it. The gas is dissolved under a pressure of two to four atmospheres and the bottle is sealed while that pressure is held, which is the whole of the manufacturing trick: gases dissolve better the harder you press them, so raising the pressure lets far more carbon dioxide stay in solution than the liquid could otherwise hold. Part of it reacts with the water to give carbonic acid, H₂CO₃, and that weak acid is what supplies the sharp, slightly sour bite — the fizz is not only bubbles, it is a taste. Open the cap and the pressure above the liquid drops to atmospheric at once; the water can no longer keep that much gas dissolved, and the excess comes out as the stream of bubbles you see rising. The same behaviour explains why a warm drink goes flat faster than a cold one, since gases are less soluble in warm water. The other three gases in this question are all real and all industrially important, and every one of them would be dangerous to drink. Two points on the printing: the paper says 'Which one of the following gas is used', where the sense wants 'gases', and it is worth noticing that the formula in the correct option is the only one in the list that names a substance the body already handles — carbon dioxide is what your own respiration produces.

• (a)NH₃ — Ammonia — pungent, alkaline in water and toxic. Its familiar uses are as a refrigerant and as the feedstock for nitrogenous fertiliser, not as anything added to food.
• (b)PH₃ — Phosphine — highly poisonous and liable to catch fire on contact with air. It is used as a fumigant against insects in stored grain.
• (c)COCl₂ — Phosgene — a chemical-warfare agent of the First World War, lethal at low concentrations. Industry uses it in making polycarbonates and isocyanates, under containment.

The amount of a gas that will dissolve in a liquid rises in proportion to the pressure of that gas above the liquid, a relationship known as Henry's law, and falls as the temperature rises. Carbonation is that law applied commercially: chill the water, press carbon dioxide in, seal the container. Some of the dissolved gas forms carbonic acid, which is why carbonated water is mildly acidic and why the drink tastes sharp. Releasing the seal drops the pressure, the solution becomes supersaturated, and gas leaves it as bubbles until a new equilibrium with the air is reached.

Odd-one-out questions on gases are usually decided by elimination on safety rather than by knowing the manufacturing process. Ammonia, phosphine and phosgene are all either toxic or violently reactive, so a candidate who recognises even two of the three formulae can close the question without thinking about solubility at all. The formulae themselves are the real difficulty, and they are worth learning as a set, because COCl₂ in particular is easy to misread as an innocuous chloride. There is also a piece of history sitting behind this item: Joseph Priestley described a way of impregnating water with fixed air, as carbon dioxide was then called, in 1767, and that is the ancestor of the entire soft-drink industry.

• Carbon dioxide is dissolved into soft drinks under pressure, typically of the order of two to four atmospheres.
• Part of the dissolved gas forms carbonic acid, H₂CO₃, which gives carbonated water its sharp taste and mild acidity.
• Gas solubility rises with pressure and falls with temperature, which is why an opened or warm drink loses its fizz.
• Joseph Priestley published a method of impregnating water with fixed air in 1767, the origin of manufactured carbonated water.
• Solid carbon dioxide is dry ice; it sublimes at about −78·5 °C at atmospheric pressure instead of melting.
• Phosgene, COCl₂, was used as a chemical weapon in the First World War; phosphine, PH₃, is a grain fumigant.

• Reading COCl₂ as an ordinary chloride rather than as phosgene.
• Assuming the fizz is only physical; the dissolved gas also forms carbonic acid, which is what the sharpness tastes of.
• Confusing the acidity of a cola with the carbonation — cola also contains added phosphoric and citric acids.

As a which-gas item like this one, as a test-for-the-gas question using limewater, or from the other direction — what dry ice is, or why a bottle of drink foams when it is opened.

• practice — not a real PYQ

  A sealed bottle of carbonated drink fizzes as soon as the cap is removed. The immediate reason is that

  • (a)the temperature of the liquid rises sharply
  • (b)the pressure above the liquid falls, so less gas can stay dissolved
  • (c)carbonic acid is created at the moment of opening
  • (d)air dissolves into the liquid and displaces the water

  Answer(b) the pressure above the liquid falls, so less gas can stay dissolved — the excess carbon dioxide leaves as bubbles.

• practice — not a real PYQ

  Carbon dioxide passed through limewater turns it milky. The white precipitate responsible is

  • (a)calcium hydroxide
  • (b)calcium carbonate
  • (c)calcium oxide
  • (d)calcium bicarbonate

  Answer(b) calcium carbonate — and passing excess carbon dioxide clears the milkiness by forming soluble calcium bicarbonate.