FAA Powerplant Written Practice Test

Specific gravity compares the weight of a substance to what?

1. An equal volume of oil

2. An equal volume of mercury

3. An equal volume of distilled water at a specific temperature

4. The average weight of air

The correct answer is: An equal volume of distilled water at a specific temperature

Specific gravity is a measure that compares the weight of a substance to the weight of an equal volume of distilled water at a specific temperature, typically at 4 degrees Celsius when water is at its maximum density. This comparison is crucial because it provides a standardized reference point, allowing for the determination of whether a substance will float or sink in water. The specific gravity is dimensionless since it is a ratio of the densities of the two substances, and it helps in identifying substances and understanding their behavior under various conditions. In aviation and other applications, knowing the specific gravity of fuels and oils is essential because it influences performance characteristics and fuel management. The use of distilled water as a reference is standard practice because it provides consistency across measurements, ensuring that assessments and comparisons are reliable and have a defined context.