When measuring the specific gravity of a substance, what standard material is often used?

• A. Air
• B. Mercury
• C. Water
• D. Alcohol

Answer: (C)

The correct answer is water, which serves as the standard reference material for measuring the specific gravity of substances. Specific gravity is defined as the ratio of the density of a substance to the density of water at a specific temperature, typically at 4 degrees Celsius, where water reaches its maximum density. Since water has a specific gravity of 1, it provides a clear benchmark for comparing the densities of other materials.

When a substance has a specific gravity greater than 1, it is denser than water and will sink when placed in it. Conversely, a substance with a specific gravity less than 1 is less dense than water and will float. This standardized measurement is critical in various applications, including hazardous materials handling, as it helps in identifying materials and assessing their behavior in different environments. Water's ubiquity and unique properties make it the most practical and commonly used reference for this purpose.