Specific gravity is the ratio of a material's density to the density of a standard material, such as water at a specified temperature, such as, 60 degrees F., or gases at standard conditions of temperature and pressure. Specific gravity is a convenient concept because it is usually easier to measure than density, and its value is the same in all systems of units.
Finding The Specific Gravity of a Substance
Depending on the substance, the way specific gravity is determined is by taking a measurement of the density of the substance in comparison to the weight of water. Temperature and atmospheric pressure must be taken into consideration in order to establish accuracy. When taking a sample, for instance, the specific gravity of the water should be taken under the same conditions as the density of the substance. To correctly calculate the specific gravity of a substance, the equation is: Density equals mass/volume. Measuring density is a complex process because the temperatures of the substances can change. Thus, it is important to measure density before finding the specific gravity of a substance.
Tips on Finding Specific Gravity of a Substance
The specific gravity of water is a known factor at 4 degrees C. This is an indication that the temperature of water is slightly above freezing. Measure your substance at this same temperature. Use mass and volume to calculate density as the formula D=m/v indicates. Mass should be calculated before volume. If the substance is not volatile, temperature changes can alter volume more than temperature changes alter mass. Units on water density should match units of mass and volume for correct conversion. Divide the specific weight of the substance with the specific weight of water. This calculation will result in a ratio without units. The units of water and substance are made compatible, and so the ratio will be the final result. The ratio makes the density of the substance and the density of water relative.
Specific Gravity in Chemical Substances
In most large chemical-producing companies the certificate of analysis includes the water content, specific gravity and molecular weight of each specific chemical compound among other relevant factors. The mass balance sheet contains the specific data on the raw materials and chemical proportions that go into creating the compound. The mass balance sheet, depending on the chemical substances involved, may include the data that conclusively determine density and specific gravity. This is usually a laundry list for use by in-house chemistry research and development groups and also as a guide should questions arise on the chemical make-up of a specific compound. Specific gravity of substances can have varying degrees of effect on compound use when factored into other compounds. It can alter viscosity and other elements of the formula in specific compounds.