What does specific gravity (vapor density) measure?

Specific gravity, often referred to in the context of vapor density when dealing with gases, quantifies the ratio of the molecular weight of a gas to that of air at the same temperature and pressure. This measurement is crucial in many applications, including fire investigations and risk assessments, as it helps determine whether a gas will rise or settle in the environment.

Since air has an average molecular weight of approximately 29 g/mol, the specific gravity of a gas can indicate how heavy it is relative to air. If the specific gravity is less than 1, the gas is lighter than air and will rise. Conversely, if the specific gravity is greater than 1, the gas is heavier than air and will tend to settle closer to the ground.

The options that pertain to the weight of a gas compared to a liquid or to the density of vapor under pressure do not accurately describe what specific gravity measures. Additionally, the temperature of vaporization relates to the point at which a substance changes from a liquid to a vapor, which is not what specific gravity is focused on. Therefore, the key takeaway is that specific gravity (or vapor density) specifically measures the ratio of the molecular weight of a gas to that of air, which is essential for understanding its behavior