Specific Gravity: Application, Formula and Sample Questions

Specific gravity refers to the ratio of density of any substance to the density of the reference substance. It is commonly called as ‘Relative Density’. Calculation of specific gravity help us to understand whether an object will float or sink in the reference substance. It is a dimensionless quantity since it includes comparison between two same physical quantities. The symbol commonly used to denote specific gravity is SG or G.

\(SG = \frac{\rho_{substance}}{\rho_{reference}}\)

where, \(\rho\) is the density

Read Also: Difference Between Density and Specific Gravity

Key Terms: Specific Gravity, Relative Density, Byoyancy, Density of Water, Pressure, Apparent Specific Gravity, Ideal Gas Law

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What is Specific Gravity?

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The specific gravity, also known as the relative gravity, is a dimensionless quantity that can be explained as the ratio of the density of a substance to the density of the reference at a specified temperature. It also accounts for Buoyancy, which is the ability of an object to float in water or oil. The temperature plays a very vital role in determining the relative density of any substance. 

If the substance has relative density less than the specific gravity of the liquid, it will float on the liquid, for example, helium balloons will rise in the air, the oil floats on water, and lead will float on mercury. The relative density of liquids is usually measured taking water as the reference and for gases, air is used as the reference at room temperature.

• If the relative density of a substance is less than 1, then it indicates that the substance is less dense than the reference.
• If the value is more than 1, the substance is more dense than the reference and a value of 1 indicates equal densities of both the substance and the reference.

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Specific Gravity of Liquids 

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To determine the specific gravity of liquids, the density of water at 4ºC is taken as the reference. This is because at 4ºC water reaches its maximum density. In SI unit, the density of water is considered to be 1000 kg/m³ or 1 g/cm³. Substances with relative density less than 1 will float on water and those having more than 1 will sink in it. 

The true specific gravity of a liquid can be expressed as,

\(SG_{true}=\frac{\rho_{sample}}{\rho_{water}}\)

The density of water in various units are:

• 1000 kg/m³
• 1 g/cm³
• 62.43 pounds per cubic foot
• 0.036 pounds per cubic inch

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Specific Gravity of Gases 

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In case of gases, the calculation for the specific gravity is generally done taking air as the reference. For gases, it is defined as the ratio of the density of the gas to the density of air at standard conditions (1 atm pressure at 60ºF). In gases, relative density can be used as an indication of the composition of the gas. The value of specific gravity for natural gas ranges from 0.55 to 0.87 depending upon its composition. 

If ideal gas law is considered, the specific gravity of a gas can be defined as the molecular weight of a gas divided by the molecular weight of the air. 

\(SG = \frac{M_{gas}}{M_{air}}\)

The table below contains values of specifc gravity of some gases:

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Factors Affecting Specific Gravity 

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The value of relative density of a substance changes with respect to the change in temperature and pressure. 

• Temperature – Density is defined as the mass per unit volume of a substance. Any change in the volume of the substance will change the density of that substance. Volume of a substance changes if we increase or decrease the temperature. Hence, specific gravity changes with temperature. 
• Pressure – Pressure is directly proportional to the specific gravity. If pressure is increased, the intermolecular spaces between the molecules decreases, increasing the density of the substance. 

Read More:

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Applications of Specific Gravity 

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The concept of specific gravity is used in various applications. Some of the major applications are listed below:

• This can be used to determine the purity of the gem by comparing its specific value to the specific value of another high purity gem. With this method, the purity of the gem can be calculated very fast and also effectively.
• It is also being used by the mineralogist and geologist for determining the mineral content of a rock.
• Urinary specific gravity is a measure of the concentration of solutes in the urine. This test is a routine test of a Urinalysis.
• The relative density concept can be used by the chemist to check on the progress of the reaction and concentration of the solutions.
• This is also used by the auto mechanics, for testing the battery fluid and antifreeze.

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Things to Remember 

• Specific gravity is the ratio of the density of the object to the density of the reference substance.
• It can also be termed as Relative Density.
• Water at 4ºC is generally used as the reference for calculating specific gravity of the liquids.
• The value of specific gravity depends mainly upon temperature and pressure.