What distinguishes a gas from a vapor?

The distinction between a gas and a vapor primarily revolves around their states under standard conditions. A gas is defined as a substance that exists in the gaseous state at room temperature and standard atmospheric pressure. This means that there are substances, like oxygen and nitrogen, which are gases at normal conditions.

In contrast, vapor refers to the gaseous phase of a substance that is typically a liquid or solid at room temperature. For instance, water is a liquid under standard conditions but can become water vapor when it evaporates. Therefore, identifying that a gas is a substance that exists as a gas in its normal state accurately captures what differentiates gases from vapors.

This understanding aligns with the nature of gases versus vapors—gases are not dependent on temperature changes to exist in that state, while vapors are associated with phase transitions from liquids or solids to gases. The chosen answer provides a fundamental clarification of the terms and their proper usage in scientific discourse.