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June 20, 2010 at 1:24 PM DENSITY Although steel is a very strong building material, it is not used to build aeroplanes. Instead, aluminium in the form of an alloy is used because of its low density (so that it can fly) and high strength (to weight ratio). Density, ρ (Greek letter 'rho' pronounced as 'row'), is the mass per unit volume of a material. The SI unit for density is kilogram per cubic metre (kg/m^3). It is also common to use the unit grams per cubic centimetre (g/cm^3), where 1g/cm^3 = 1000kg/m^3. Density of all matter depends on temperature; the density of a mixture may depend on its composition, and the density of a gas on its pressure. Using the ρ symbol for density, m for mass and V for volume, the expression for density is: ρ = m/V  The densities of some common substances are given below. Using tables such as this, chemists are able to determine if samples of such substances are pure by testing their densities. To measure the density of a liquid, a hydrometer is used.    How does a hydrometer works? Hydrometer is specially designed to float in a liquid and measure its density. If the hydrometer 'float higher', it indicates that the liquid has a higher density. When an object is placed in a liquid of a greater density, the object floats. On the other hand, if it is placed in a liquid of lower density, it sinks. For example, a copper sphere of density 8.92g/cm^3, sinks in water of density 1g/cm^3 but floats in mercury of density 13.6g/cm^3. The denser the liquid, the higher an object will float in the liquid.  |