Used to determine vapor density.

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Used to determine vapor density.

Vapor density is a measure of how heavy a vapor is compared with air, and at the same temperature and pressure, a gas’s density is proportional to its molecular weight. In other words, the heavier the molecule, the denser the vapor. Since vapor density is defined relative to air, knowing the gas’s molecular weight lets you estimate whether it will be lighter or heavier than air and predict where it will travel or accumulate. Boiling point describes a substance’s volatility, not how heavy its vapor is. Pressure can change actual density, but vapor density as a concept is referenced to air under standard conditions, so it’s the molecular weight that sets the comparative density. Specific gravity compares density to water, not air, so it’s not the right basis for vapor density.

Vapor density is a measure of how heavy a vapor is compared with air, and at the same temperature and pressure, a gas’s density is proportional to its molecular weight. In other words, the heavier the molecule, the denser the vapor. Since vapor density is defined relative to air, knowing the gas’s molecular weight lets you estimate whether it will be lighter or heavier than air and predict where it will travel or accumulate.

Boiling point describes a substance’s volatility, not how heavy its vapor is. Pressure can change actual density, but vapor density as a concept is referenced to air under standard conditions, so it’s the molecular weight that sets the comparative density. Specific gravity compares density to water, not air, so it’s not the right basis for vapor density.

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