The method used to collect gas in the laboratory depends on its? I. boiling point  II. density  III. smell  IV. solubility in water

Analyze the question: The question asks which properties of a gas determine the method used to collect it in the laboratory.

Concept: There are three main methods of collecting gases:

1. Downward delivery (upward displacement of air): used for gases denser than air, e.g. Cl₂, CO₂, HCl, SO₂.
2. Upward delivery (downward displacement of air): used for gases less dense than air, e.g. NH₃, H₂.
3. Over water (downward displacement of water): used for gases that are insoluble or only slightly soluble in water, e.g. H₂, O₂, N₂, CO, NO.

Some gases, like HCl and NH₃, are very soluble, so they cannot be collected over water. The choice of method therefore depends on:

• Density (relative to air), which decides between upward and downward delivery.
• Solubility in water, which decides whether the gas can be collected over water.

Checking each property:

• I. Boiling point: it does not decide the collection method. It matters when liquefying a gas or in fractional distillation, not in routine collection. ✗
• II. Density: ✓
• III. Smell: smell is only used to identify a gas or warn of its presence. It does not affect how the gas is collected. ✗
• IV. Solubility in water: ✓

So the correct combination is II and IV only.

Exam tip: Remember "Density and Solubility decide the Delivery method." Whenever a question asks about collecting gases, think of density (air displacement) and solubility (over water). Smell and boiling point are distractors.