# Density and relative density

Density and relative density

One often hears the expressions, “as light as a feather” and “as heavy as lead”. Equal volumes of different substances vary considerably in mass. Aircraft are made chiefly from aluminium alloys, which provide a structure as strong as steel but which, volume for volume, weigh less than half as much. In physics we refer to the lightness or heaviness of different materials by the use of the word density. (The density of a substance is defined as its mass per unit volume.One way of finding the density of a substance is to take a sample and measure its mass and volume. The density may then be calculated by dividing the mass by the volume. The symbol used for density is the Greek letter p (rho). densit.y = mass. (in  ppropnat.e. units: see beIow) The densities of all common substances, solids, liquids and gases, and all chemical elements have been determined and are to be found listed in books of physical and chemical constants.

The SI unit of density is the kg/m ‘, but when densities are being measured in the laboratory it is generally most convenient to work in grams and cubic centimetres, thus obtaining a result expressed in g/cm ‘. There is no difficulty in converting
this to SI units (kg/rn ‘). Water happens to have a density of about I g/cm3 or 1000 kg/rn ‘ owing to the fact that the kilogram was originally intended to have the same mass as 1000 crn ‘ of water at 4 “C. Mercury is a metal which is a liquid at ordinary temperatures and it  has the very high density of 13.6 g/cm3. It is a very useful substance in scientific laboratories and plays a part in many experiments. When one lifts up a bottle of mercury for the first time one is surprised by its weight.