How to Calculate the Specific Gravity of Water: The Formula You Need

Specific gravity is a dimensionless quantity that represents the ratio of the density of a substance to the density of water at a specific temperature. For water, the specific gravity is set at 1.0 at 4°C, which is its densest state. This standard helps in comparing the densities of other substances.

Specific Gravity Formula for Water

The formula to calculate specific gravity is:

Specific Gravity (SG)=Density of SubstanceDensity of Water\text{Specific Gravity (SG)} = \frac{\text{Density of Substance}}{\text{Density of Water}}Specific Gravity (SG)=Density of WaterDensity of Substance​For water, the density is approximately 1 g/cm³ at 4°C. Therefore, at this temperature and standard atmospheric pressure, the specific gravity of pure water is exactly 1.

Step-by-Step Process to Calculate Specific Gravity of Water:

1. Understand Water’s Density:At 4°C, the density of water is 1 g/cm³ or 1000 kg/m³. As the temperature rises or falls, the density changes slightly, affecting its specific gravity. For instance, at 20°C, water’s density drops to around 0.998 g/cm³​.
2. Measure the Density of a Liquid:To compare the density of another liquid to water, measure the mass and volume of the liquid to find its density, then use the specific gravity formula.
3. Use a Hydrometer:A common tool to measure the specific gravity of liquids, including water, is the hydrometer, which floats in the liquid and indicates its relative density​.
4. Account for Temperature Variations:Always factor in temperature when calculating water’s specific gravity, as its density decreases as temperature rises​.

Example Calculation:

If the density of a substance is 980 kg/m³ at 20°C, the specific gravity would be:

Specific Gravity=9801000=0.98\text{Specific Gravity} = \frac{980}{1000} = 0.98Specific Gravity=1000980​=0.98This means the substance is slightly less dense than water at 20°C.

Applications of Specific Gravity in Water:

• Engineering: Used to design ships and submarines.
• Oceanography: Helps in understanding water properties like salinity.
• Daily Life: Plays a role in buoyancy, which affects swimming and other water activities​.

FAQ

1. What is the specific gravity of water? The specific gravity of pure water is 1.0 at 4°C, which is its densest state.
2. How does temperature affect water’s specific gravity? As the temperature increases, water’s density decreases, leading to a lower specific gravity.
3. What tools are used to measure specific gravity? Tools like the hydrometer and pycnometer are commonly used for measuring the specific gravity of water and other liquids.
4. Why is the specific gravity of water important? It serves as the reference point for comparing the densities of other liquids and plays a key role in fluid mechanics and engineering applications.
5. Can specific gravity change due to dissolved substances? Yes, dissolved salts or other substances increase the density of water, raising its specific gravity, as seen in seawater compared to freshwater.

By understanding and calculating the specific gravity of water, we gain insights into fluid properties, buoyancy, and engineering designs that rely on water’s density​.