The relative density of a substance refers to its density relative to what?

The relative density of a substance is a measure of its density in comparison to the density of a standard reference material. Water is commonly used as this reference substance because it has a well-established density of approximately 1 g/cm³ at standard temperature and pressure. When we say a substance has a relative density greater than 1, it means that the substance is denser than water. If a substance has a relative density less than 1, it indicates that it is less dense than water.

This comparison helps in understanding the buoyancy of the substance when placed in water, which is crucial in various scientific and practical applications, such as determining whether a substance will float or sink. For these reasons, water is the standard reference point for relative density assessments, making it the correct answer for this question.