The gas that can be collected by downward displacement of air is

Analyze the question: "Downward displacement of air" means the gas is collected in an upright jar, with the gas entering from the top and pushing the air out from the bottom. This only works if the gas is less dense than air.

Explain the concept: Collection method depends on density and solubility:

• Downward delivery (upward displacement of air): for gases denser than air. The gas sinks to the bottom of the jar.
• Upward delivery (downward displacement of air): for gases lighter than air. The gas rises and collects at the top while air is pushed down and out.

Note that "downward displacement of air" refers to the air being pushed downward, so the gas must be lighter than air.

The average molar mass of air is about 29 g/mol, so we compare each gas:

| Gas | Molar mass | Compared with air | |---|---|---| | Chlorine, Cl₂ | 71 | Denser | | Sulphur (IV) oxide, SO₂ | 64 | Denser | | Carbon (IV) oxide, CO₂ | 44 | Denser | | Ammonia, NH₃ | 17 | Lighter |

Solution:

• Cl₂, SO₂ and CO₂ are all heavier than air, so they are collected by upward displacement of air (downward delivery).
• Ammonia (17 g/mol) is lighter than air. It is also very soluble in water, so it cannot be collected over water. The suitable method is therefore downward displacement of air, with the jar held upside down.

Answer: D. ammonia.

Exam tip: Remember that ammonia is the common lighter-than-air gas (like hydrogen) in the syllabus. Ammonia is collected with an inverted jar, and it is dried with quicklime (CaO), not concentrated H₂SO₄ or CaCl₂, because it reacts with those. If a gas has a molar mass below 29, think "upward delivery, jar upside down."