6) Physical properties of glass

1. Structural properties of glass:
a) Density and Molar volume:
Density ? is defined as mass per unit volume. Appropriate units are g/cm3 (or g/cc) in the cgs system and kg/m3 in the SI system.
Relative density is defined as density with respect to water at 4°C and is, hence, unitless. Because the density is inversely proportional to the volume, a change ?T in temperature changes the density by –3? ?T, where ? is the linear thermal expansion coefficient. Molar volume VM is the volume of one mole formula weight and is computed by:
VM = M/?
where M is the mol weight in grams with the formula of glass expressed on a one-molecule basis (or on a one-atom basis for elemental glasses, such as the chalcogenides). Thus, a glass of the formula Na2O•Al2O3• 6SiO2 should be first written as 1/8Na2O • 1/8Al2O3 • 6/8SiO2 to obtain the value of M.

Dependence on Composition:
Density of silica glass is 2.20 g/cm3 at room temperature. There is no detectable difference observed in density of silica regardless of the different methods of production. Of the various forms of crystalline silica, the density of silica glass is closest to that of ?-cristobalite (high-temperature form), which is 2.25 g/cm3. Since the density of ?-quartz is 2.65 g/cm3, it is apparent that large structural changes occur during the melting of sand. Addition of alkalis to silica increases the density steadily: The alkali ions go inside the interstices as NWM, taking up holes. Heavier alkali ions generally are more effective in increasing the density. Likewise, heavy elements such as Ba, Cd, Pb, and Bi bring large increases in the density.
Because density can be measured readily and accurately to the third decimal place, and because it is extremely sensitive to composition, density charts are