Define “dry air”: is it a mixture of vapors, gases, or something else? Choose the most accurate description used in chemical engineering.

Introduction / Context:
Air handling and psychrometrics rely on a clear definition of “dry air.” In calculations, distinguishing between gases and vapors matters for how we treat species and partial pressures. Dry air is the reference state before adding water vapor to obtain moist air.

Given Data / Assumptions:

• “Dry air” excludes water vapor.
• Main constituents: nitrogen, oxygen, argon, carbon dioxide (in trace), and other noble gases.

Concept / Approach:
Each constituent of dry air is treated as a gas under ambient conditions. “Vapor” generally refers to the gaseous phase of a substance that is liquid at room conditions (e.g., water vapor). Since water vapor is specifically excluded, dry air is correctly described as a mixture of gases (N2, O2, Ar, etc.). When water is present, we speak of “moist air,” a gas–vapor mixture where water is the vapor component.

Step-by-Step Solution:

Verification / Alternative check:
Textbook psychrometric charts start with dry air (gas mixture), then overlay water vapor properties; the combination is treated via Dalton’s law as a gas–vapor mixture only when water is included.

Why Other Options Are Wrong:

• (a) Vapor implies presence of a condensable like water; excluded in “dry” air.
• (c) Both is incorrect by definition of dry air.
• (d) “Neither” contradicts classification.
• (e) Not a single pure gas; it is a mixture.

Common Pitfalls:
Calling moist air “a mixture of gases”; strictly it is a gas–vapor mixture because water behaves as a vapor relative to room temperature.

Final Answer:
gases