Which statement correctly differentiates a gas from a vapor?

Which statement correctly differentiates a gas from a vapor?

• A. Gas is a substance in the gaseous state at ambient temperature; vapor is the gaseous phase of a substance that is normally a liquid or solid at room temperature; exposure risk depends on concentration, dispersion, and release rate.
• B. A gas is always heavier than air; a vapor is lighter than air; exposure risk is independent of concentration.
• C. A gas is only produced by organic compounds; vapor is only produced from inorganic compounds.
• D. A gas becomes a vapor only at high altitudes.

Answer: (A)

The key idea is that a gas is a substance in the gaseous state at ordinary conditions, while a vapor is the gaseous form of a substance that is normally a liquid or solid at room temperature. This matters because hazard assessment focuses on how much of that substance is present in the air, how it disperses, and how fast it can be released into the environment. A gas like helium is inherently gaseous at ambient conditions, whereas a liquid like water becomes water vapor when it evaporates. Substances can also exist as vapors when they are normally liquids or solids, which is why we talk about vapors in terms of evaporation or boiling into the air. Other statements that try to tie gases to a fixed density relative to air, or to inorganic versus organic, or to altitude, don’t reflect how the terms are used in hazmat contexts.

The key idea is that a gas is a substance in the gaseous state at ordinary conditions, while a vapor is the gaseous form of a substance that is normally a liquid or solid at room temperature. This matters because hazard assessment focuses on how much of that substance is present in the air, how it disperses, and how fast it can be released into the environment. A gas like helium is inherently gaseous at ambient conditions, whereas a liquid like water becomes water vapor when it evaporates. Substances can also exist as vapors when they are normally liquids or solids, which is why we talk about vapors in terms of evaporation or boiling into the air. Other statements that try to tie gases to a fixed density relative to air, or to inorganic versus organic, or to altitude, don’t reflect how the terms are used in hazmat contexts.