When we go out in the summer to enjoy the pools and rivers, we must constantly double-check that all of our equipment and safety precautions are in place and operational.

What is Specific Gravity?

What is Specific Gravity

When we go to the river and board our boats, we make sure there are no leaks and that we are wearing our life jackets. We make sure that young children who are still learning to swim have their floaters on or swim with them in a swim ring to ensure that they do not sink.

Why do both boats and swimming rings float?

One of the parameters that determine whether an object will sink or float in water is what is known as specific gravity.

The specific gravity of an object is the ratio of its density to that of a reference substance. The specific gravity can inform us whether an object will sink or float in our reference substance dependent on its value.

Water is commonly used as a reference substance since it has a density of 1 gram per milliliter or 1 gram per cubic centimeter.

The formula for Specific Gravity

Specific gravity can be expressed mathematically as:

SG= ρsubstance/ρwater 

Here, the Greek letter Rho is used to denote density in this case.

The specific gravity of any substance is defined in terms of water at 4°C in general. This temperature is taken into account since the density of water at that temperature is 1000kg/m3 (1g/cm3).

The specific gravity value of any material can also be calculated using the density of water in different units. 62.43 pounds per cubic foot and 0.036 pounds per cubic inch are the various densities of water.

Specific Gravity = Density of the object/Density of water

   = ρobject/ρH2O 

What is the Specific Gravity of water?

Every material thing, whether liquid, solid, or gas, has a characteristic density that you can calculate by multiplying the mass and volume of the object. The calculation yields a number of mass units divided by volume units.

Scientists devised a metric called specific gravity, which is density divided by the density of water at 4 degrees Celsius and at atmospheric pressure, to avoid having to express the units, which can be laborious. Water has a specific gravity of 1 at that temperature and pressure, however, water with impurities has a specific gravity that varies slightly from 1.

Specific Gravity of other Substances

You could compute the specific gravity of any substance if you knew what specific gravity meant. Based on these parameters, specific gravity can be used in a variety of ways. In general, gases have the lowest specific gravity, whereas solids have the highest. Dry air, for example, has a specific gravity of 0.0013.

The presence of moisture in the air, on the other hand, will raise the specific gravity value. Alcohol has a specific gravity of 0.82. Carbon dioxide has a specific gravity of 0.00126. Rubber and wood oak will be exceptions because they are solids with a specific gravity less than water (0.96 and 0.77, respectively).

How to Determine Specific Gravity?

You divide the density of a material by the density of water at 4 degrees Celsius to get the dimensionless measure known as specific gravity. The density of the material and the density of water must both be given in the same units for this calculation to work. The density of water in various units is shown below:

  • 1 gram per cubic centimeter
  • 1000 kilograms per cubic meter
  • 62.43 pounds per cubic foot
  • 0.036 pounds per cubic inch

The density of lead, for example, is 11.36 grams per cubic centimeter (g/cc). To find the specific gravity of lead, divide by the density of water in the same unit.

Lead has a density of 0.41 pounds per cubic inch (lbs./in3) when measured in different units. To get the same figure, i.e., 11.36 divide this by the density of water in the same units – 0.036 lbs./in3.

Impurity Concentration Measured by Specific Gravity

Because scientists utilize water at 4 degrees Celsius to estimate specific gravity, it follows that its specific gravity is 1. A water sample was taken at a different temperature or pressure, or one that contains impurities has a somewhat variable density.

You can gather information about a sample by measuring its density and dividing it by the density of pure water at 4 degrees Celsius to get its specific gravity. If the sample’s specific gravity is greater than 1, the temperature is 4 degrees Celsius, and the pressure is atmospheric, the sample contains impurities.

The specific gravity of a water sample can tell you its concentration if you know which impurity it contains.

Factors Affecting Specific Gravity

As previously stated, specific gravity is a measurement of an object’s density in relation to the density of water at a certain temperature and pressure. The density of items and liquids is affected by changes in pressure and temperature, which affects the specific gravity of objects and liquids. Specific gravity is calculated using standard temperature and pressure. If the external factors are not controlled, the specific gravity will change.

Applications of Specific Gravity

Specific gravity has a wide range of uses, some of which are listed below:

  • The purity of a gem can be assessed by comparing its specific gravity to that of another gem that has already been measured at a high purity level. This enables a fast assessment of the gem’s worth.
  • Mineralogists and geologists utilize specific gravity to determine the mineral content of a rock.
  • The concentration of solutes in the urine is measured by urine-specific gravity. The test is a standard component of a urinalysis.

Effects of Specific Gravity

As we know, the density of an object or liquid in comparison to the density of water at a precise temperature and pressure is known as specific gravity. Changes in temperature and pressure have an impact on the density of objects and liquids, and thus on their specific gravity.

Temperature and pressure will modify the specific gravity of a substance or liquid, including water, the reference liquid. As a result, while calculating specific gravity, a standard temperature and pressure are utilized. Specific gravity will change if those external influences are not controlled.

Water is an excellent example of this idea. Water is less dense when frozen than when it is at 39 degrees Fahrenheit. When water is heated over 39 degrees Fahrenheit, it becomes less dense.

  • Water has a density of 915 kg/m cubed at 32 degrees Fahrenheit.
  • Water has a density of 1000 kg/m cubed at 39 degrees Fahrenheit.
  • Water has a density of 971.8 kg/m cubed at 176 degrees Fahrenheit.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.