The specific gravity of a substance is a measure of its density compared to the density of another reference substance, typically water. The specific gravity of water is defined as 1 because it is used as the reference substance for comparison.
The specific gravity of a substance is calculated by dividing its density by the density of the reference substance. Since the density of water is already known, dividing the density of any substance by the density of water will give a specific gravity value relative to water.
Water is often chosen as the reference substance for specific gravity calculations because it is a common and well-known substance with a relatively constant density under standard conditions (typically at 4 degrees Celsius or 39.2 degrees Fahrenheit).
By convention, the specific gravity of water is set to 1, so any substance with a specific gravity greater than 1 is denser than water, and any substance with a specific gravity less than 1 is less dense than water. This convention simplifies comparisons and calculations involving the densities of various substances.