# Density measurement of solids

The density of solids is a physical quantity characterizing its mass per unit volume. Density derives from this quantity: it is the ratio between its density and the density of water. Densities of solids are commonly expressed in kg/m3 or g/cm3.

## Density measurement methods

A simple approach consists of measuring the volume of solid samples by direct measurement with suitable measuring instruments: calipers, etc. This technique is suitable for measurements at room temperature on samples with simple geometries (cubes, cylinders, etc.). It gives results whose precision is sometimes sufficient. On this type of sample, a density value at ambient temperature coupled with dilatometry tests also make it possible to calculate density values at temperatures other than ambient temperature.

While a liquid will take up all of the space it occupies, solids can have empty spaces. This is called their porosity. These empty spaces may or may not lead to the outside. It is thus advisable to specify the density measured according to whether or not it takes account of these porosities. We will then sometimes speak of apparent density (volume of emerging porosities included) or skeletal (volume of non-emerging pores not included).

For solids of non-classical shapes, liquid pycnometry can also be used: this approach consists of placing a piece of solid sample in a pycnometer and then filling the latter with a fluid of known density. The solid sample must of course not be soluble in the fluid used. The fluid will thus occupy all the volume not occupied by the solid. This makes it possible to deduce therefrom by subtraction the volume of the immersed solid sample.

One of the techniques commonly used to determine the density of solids at different temperatures is gas pycnometry. This technique consists in determining the volume of a solid sample by performing gas pressure variations in an enclosure containing the sample. This technique is particularly suitable for powders and granules.