Which of the following best describes specific gravity?

Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water, at a specified temperature and pressure. This concept is critical in various fields, including chemistry and engineering, as it provides insight into whether a substance will float or sink in water, helps in the identification of substances, and allows for the comparison of different materials.

The choice relating to the density ratio to water is particularly relevant because water is commonly used as the baseline due to its unique properties and availability. For example, a specific gravity greater than 1 indicates that the substance is denser than water and will sink, while a specific gravity less than 1 suggests that the substance is less dense and will float.

The idea of comparing a substance's weight to air, measuring water solubility, or describing gas pressure, while relevant in certain contexts, does not accurately capture the essence of specific gravity, which is fundamentally about density comparison with water. Therefore, identifying specific gravity as the density ratio of a substance to water aligns directly with its definition and practical application.