The reason specific gravity is often cited rather than density is that:
Specific gravity has units that are consistent with the SI (metric) system.
The numerical value of density depends on the mass and volume units chosen, whereas specific gravity is unit-independent.
The specific gravity of bodily fluids is always close to 1.
If the specific gravity of a substance is greater than 1, you know it will float on water.
The correct answer and explanation is :
The correct answer is:
The numerical value of density depends on the mass and volume units chosen, whereas specific gravity is unit-independent.
Explanation (approx. 300 words):
Specific gravity is often preferred over density in many scientific and industrial contexts because it is a dimensionless quantity — meaning it has no units. This makes it especially convenient when comparing materials without worrying about the unit systems being used.
Density is defined as mass per unit volume (e.g., kilograms per cubic meter or grams per cubic centimeter). Since both mass and volume have units, the calculated density value depends on the units chosen. For instance, the density of water is approximately 1000 kg/m³ in SI units, but 1 g/cm³ in CGS units. These values differ numerically due to the unit conversion, which can lead to confusion or errors if the units are not properly accounted for.
On the other hand, 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, and air for gases). Because it’s a ratio of two densities, the units cancel out, leaving a pure number. For example, if a liquid has a density of 1.2 g/cm³, and water’s density is 1.0 g/cm³, its specific gravity is 1.2 — regardless of the unit system.
This unit-independence makes specific gravity a universal and intuitive way to compare how “heavy” or “light” a substance is relative to water. It also simplifies communication between scientists, engineers, and technicians who may be using different measurement systems.
While other statements in the question may hold partial truth (e.g., many bodily fluids do have SG near 1), they are not the fundamental reason why specific gravity is often cited over density. The primary advantage is its unit-independent nature, enhancing clarity and reducing errors in scientific and practical applications.
