In this post we are going to learn about the physical significance of specific gravity. Further in the post we will learn most common method to determine the specific gravity of soil.

## Specific gravity definition

Specific gravity is the relative term. Basically it is defined as the ratio of density of and soild to the density of reference material. In case of soil this reference material is **water**. So, if you want to write the expression it will be like:

Specific gravity of soild, \(G_s\)=\(\frac{\rho_{solid}}{\rho_{water}}\)

## Pycnometer method

Specific gravity of solid is very important physical property. It is used to determine other basic properties of the soil. To determine the specific gravity of solid in the laboratory pycnometer method is used. Pycnometer is from the Greek puknos, meaning “density”, also called pyknometer or specific gravity **bottle**. Steps involved to measure the specific gravity from the pycnometer is as follows:

- Take the empty pycnometer bottle and take its weight say weight is \(W_1\).
- In the next step take 300g of soil. Put this soil in oven for drying and cool it in desiccator.
- Keep this soil in pycnometer bottle and take the weight \(W_2\).
- Fill the pycnometer with water. While filling the bottle stir it with glass rod so that no voids will be there.
- Take the weight \(W_3\) again.
- Now empty the the pycnometer bottle, clean it and dry it.
- Fill this clean and dried pycnometer bottle with water.
- Take the weight of this \(W_4\).

Also read: void ratio, permeability of soil

Calculation

Weight of dry soil, \(W_s=W_2-W_1\)

The weight of water in the observation (iii) = \(W_3-W_2\)

and weight of water in the observation (iv) = \(W_4-W_1\)

Now after this we will calculate the weight of water having the same volume as that of solids (refer fig.2) = \((W_4-W_1)-(W_3-W_2)\)

Now rearranging the terms we get – \((W_2-W_1)-(W_3-W_4)\)

Specific gravity of solids, \(G_s = \frac{W_2-W_1}{(W_2-W_1)-(W_3-W_4)}\)

In general we report the \(G_s\) at \(27^\circ{C}\). It your test temperature condition is different then you can convert it by unitary method.

## General range

In general soil have the range of 2.65 to 2.85. Soil with organic matter has the value of 2.0. If soil contains heavy metal it goes up to 3.0.