What property is measured when comparing the density of a solution to the density of water?

The property measured when comparing the density of a solution to the density of water is specific gravity. Specific gravity is defined as the ratio of the density of a substance to the density of a reference substance, typically water for liquids and solids. This measure is dimensionless; it indicates how much heavier or lighter a substance is compared to water at a given temperature.

When you calculate specific gravity, you essentially assess how a solution's density compares to that of water, which has a density of about 1 g/cm³ at 4°C. If the specific gravity is greater than 1, the solution is denser than water; if it's less than 1, it is less dense. This property is particularly useful in various applications, such as determining the composition of solutions, assessing whether a substance will float or sink in water, and in various scientific and industrial processes.

The other options are distinct concepts related to solutions' composition or concentration but do not pertain to the density comparison with water. Molarity and normality focus on the concentration of solutes, while percentage concentration refers to the ratio of solute to solution expressed as a percentage.