Certified Fire Investigator Practice Exam

What is "Specific Gravity" used to compare?

1. The weight of solid materials in air

2. The average molecular weight of a gas to that of air

3. The density of liquids compared to solids

4. The temperature of substances under pressure

The correct answer is: The average molecular weight of a gas to that of air

Specific gravity is a measurement used to compare the density of a substance—often a liquid or gas—to the density of water for liquids or air for gases. It is defined as the ratio of the density of a substance to the density of a reference substance at a specified temperature and pressure. For gases, the reference standard is air, so specific gravity of a gas effectively compares its average molecular weight to that of air. In this context, option B accurately reflects the role of specific gravity in gauging how a gas's average molecular weight compares to that of air. Understanding the specific gravity of gases is crucial, particularly in fire investigations, where the behavior of various gases in relation to air can affect fire dynamics, buoyancy, and the potential spread of smoke and combustion products. The other choices do not align with the concept of specific gravity in this application. For example, measuring the weight of solid materials in air does not involve a reference substance like water or air for comparison. Similarly, comparing the density of liquids to solids or examining the temperature of substances under pressure does not pertain to specific gravity’s defined purpose of density comparison with a reference substance.