Which of the following gasses is highly soluble in water at room temperature

Analyze the question: You are asked which gas dissolves most readily (is highly soluble) in water at room temperature.

Concept: Gas solubility in water depends mainly on polarity and on whether the gas reacts with water or forms hydrogen bonds with it.

• Ammonia (NH₃): It is polar and forms hydrogen bonds with water. It also reacts reversibly to form ammonium and hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ It is extremely soluble, about 700 volumes of NH₃ per 1 volume of water at room temperature. This is why it is used in the fountain experiment.
• Carbon (IV) oxide (CO₂): It is only slightly soluble, about 1 volume per 1 volume of water. It forms a weak carbonic acid.
• Chlorine (Cl₂): It is moderately soluble, about 2 volumes per 1 volume of water. It partly reacts to form HCl and HOCl.
• Nitrogen (N₂): It is non-polar and almost insoluble in water.

Comparing them: NH₃ ≫ Cl₂ > CO₂ > N₂ in terms of solubility (roughly). Ammonia is by far the most soluble, so it is the only one that fits "highly soluble".

Answer: A. ammonia

Tip: Remember that ammonia and hydrogen chloride are the two classic "very soluble" gases in WASSCE and JAMB. They are collected by upward or downward delivery, never over water. Gases like N₂, O₂, H₂ and CO are almost insoluble, so they can be collected over water.