SPECIFIC GRAVITY: FORMULA AND UNITS, HOW TO CALCULATE IT - CHEMISTRY - 2025

The specific gravity, also known as specific gravity or relative density, is the relationship or ratio between the density of a substance and density of another substance (it is usual in this case water is used).

These densities are often taken as guides when referring to liquids or solids. Apparent specific gravity is the ratio of the volumetric weight of one substance to the volume weight of another. Specific gravity is often used in industry, as it provides information on the concentration of solutions to use in a simple way.

The use of specific gravity can be observed mainly in the mining industry, since thanks to this method it is possible to determine what a rock is composed of.

Solutions worked with specific gravity can include various materials, such as hydrocarbons, acids, or "sugary" solutions, among others.

Thanks to specific gravity, we can deduce whether an object will sink or float in the reference substance. It is common that this substance is always water, since it is known that it has 1 gram per milliliter or 1 gram per cubic centimeter.

Formula and units

A peculiarity of specific gravity is that it has no units. This phenomenon occurs since the substances to be evaluated are the same.

The units present in both the denominator and the numerator are the same; hence, they cancel out and the result of specific gravity is without unity. For this reason, it is not defined as the absolute density of the substance but rather its relative density.

What formula is used to calculate specific gravity?

Specific gravity is calculated taking into account the following formula:

GE (ρr) = ρ substance / ρ0 reference

GE is the specific gravity, ρ substance is the density of the substance and ρ0 reference is the density of the reference material.

To calculate or measure the relative density between two materials, whether they are solid or liquid, a device known as a pycnometer is used, which is based on Archimedes' principle.

Archimedes' principle

Archimedes' principle states that when a body is partially or totally submerged in a fluid at rest, it will undergo an upward vertical impulse equal to the weight of the volumetric mass of the object in question. This force is measured in Newtons.

Since this principle studies the force exerted by an object in a flow, it is closely related to the process that takes place in specific gravity, since it expresses the relationship of two substances.

How to calculate it?

The density of a material can vary depending on its pressure or temperature; therefore, it is important to have these values ​​when calculating specific gravity.

Normally, when specific gravity is calculated, both the substance to be studied and the reference substance have all their identical units, and they must even be found at a similar temperature and pressure.

This calculation is the result of the quotient between the value of the density of the substance and the value of the reference substance (most of the time water is used).

The density of water is 1,000 g / ml at a temperature of 3.98 ° C, but a density of 1.00 g / ml can be used at a temperature of up to 25 ° C without running the risk of having an error of calculation.

Water is the most used substance since, due to its density of 3.98 ° C, the relationship between this and another substance is easier to find.

Example

When you propose to calculate the specific gravity using the density in pounds per cubic foot of a material such as mud, the calculation would be as follows:

GE = Density of mud (lb / ft³) / 62.4

DL is equal to 100 lb / ft³. So:

GE = 100 lb / ft³ / 62.4

GE = 1.60

Thanks to the specific gravity, it was observed that if a substance has a specific weight greater than 1 said material will sink in water, while if its density is below 1, the material will float.

To carry out this method, absolute precision is required; therefore, it is a rarely used method. The area where the calculation of specific gravity is used most is in highly experienced scientific fields.

Difference between specific gravity and density

Although specific gravity is also known as relative density, it is important to emphasize that it is not the absolute density of the substance.

As mentioned previously, specific gravity is the ratio of the density of one substance to the density of another substance, which most of the time is usually water. Specific gravity, or better known as relative density, has no units: being the same, they cancel out in the calculated quotient.

On the other hand, density is a quantity that expresses the relationship that exists between the mass of a material multiplied by its volume.

In the case of completely solid materials (such as iron), the density of the material does not change; that is, it is the same throughout the object.

Unlike specific gravity, density does have units: mass and volume. These units are expressed in kilograms per cubic meter (Kg x m³) according to the International System of Units. The formula to calculate the density is: ρ = m xV

Specific gravity is also known as relative density, so it can be inferred that density and specific gravity are closely linked. Relative density is a type of density.

References

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6. Specific gravity. Retrieved on May 8, 2018, from Wikipedia: en.wikipedia.org.
7. What is Specific Gravity? - Definition, Formula, Calculation & Examples. Retrieved on May 8, 2018, from Study: study.com