A gas that is collected by upward delivery is likely to be

Analyze the question: "Upward delivery" is a method of collecting gases. The question asks what property a gas must have to be collected this way.

Explain the concept: The method used to collect a gas depends on its density and solubility.

• Downward delivery (upward displacement of air): the gas is heavier than air, so it sinks and pushes the air up and out of the jar. Examples are Cl₂ and CO₂.
• Upward delivery (downward displacement of air): the gas is lighter than air, so it rises and collects at the top of the jar, pushing the air downward and out. The gas jar is held upside down. Examples are NH₃, H₂ and CH₄.
• Over water: used for gases that are insoluble or only slightly soluble in water, such as O₂, H₂ and N₂.

Solution:

1. In upward delivery, the gas enters the inverted jar from below and rises.
2. For the gas to stay at the top and force the air out at the bottom, it must be less dense than air.
3. The gas is therefore lighter than air.

Checking the options:

• A. Heavier than air: this describes downward delivery, so it is wrong.
• B. Insoluble in water: this describes collection over water, so it is wrong.
• C. Lighter than air: correct.
• D. Soluble in water: this is not what the method depends on. Ammonia is soluble and is collected by upward delivery, but the deciding property is density, not solubility.

Exam tip: Match the direction to the jar. In "upward delivery" the gas travels up, so it must be light. Remember that ammonia is collected by upward delivery because it is lighter than air and too soluble to collect over water.