Sodium metasilicate anhydrous and sodium metasilicate pentahydrate are also known as types of sodium silicate, liquid glass or water glass. They are white powders that readily dissolve in water. Both can be used as cleaners, as fire retardants, for repairing metals, for removing ink from recycled paper, for immobilising engines and to decrease the porous qualities of concrete. So what is the difference between them?
Sodium metasilicate pentahydrate is formed from the hydration of anhydrous sodium metasilicate with five water molecules. Anhydrous sodium metasilicate, which has no water associated with it, exists in a chain or polymeric structure rather than as discrete unit anions of SiO3 2-. The polymeric chain is formed when one oxygen is shared between two silicon ions. Sodium metasilicate pentahydrate are tetrahedrally shaped anions that exist as separate units, formed when anhydrous sodium metasilicate is hydrated with water.
The melting point of sodium metasilicate anhydrous is higher than the melting point of sodium metasilicate pentahydrate. Sodium metasilicate anhydrous has a melting point of 1088 degrees Celsius or 1088 degrees Celsius, compared to sodium metasilicate pentahydrate, which has a melting point of 882 degrees Celsius or 72.2 degrees Celsius. This is because of the differences in structure between the two varieties. The chains of sodium metasilicate anhydrous units have much stronger covalent bonds between them than the separate sodium metasilicate pentahydrate anions, which are held together by ionic bonds. Stronger bonds require more heat energy to separate the units, before the substance can melt.
Molar Mass and Density
With no water molecules adding to the molecular weight, anhydrous sodium metasilicate has a lower molar mass than sodium metasilicate pentahydrate at 122.06 grams per mole, compared to 212.14 grams per mole. The density is also lower for sodium metasilicate anhydrous. Sodium metasilicate anhydrous has a density of 2.4, compared to sodium metasilicate pentahydrate with a density of 2.61. Again this is due to the polymeric structure, which holds the units at a greater spacing than the ionic bonds associated with sodium metasilicate pentahydrate.
When anhydrous sodium metasilicate is hydrated to form sodium metasilicate pentahydrate, the refractive index changes from 2.4 to 2.61. This is due mainly to the increase in density. As density increases, so often does the refractive index, as the propagating light interacts with more electrons within the medium it is passing through.