Density and Brillouin-scattering measurements of hydrous andesite glasses at ambient conditions showed that dissolved water has a concentration-independent partial molar volume of 12 ± 0.5 cubic centimeters per mole and a bulk modulus of 18 ± 3 gigapascals. Dissolved as hydroxyl ions or as molecular water, water has volume properties similar to those of ice Ⅶ, the densest form of ice. These properties point to hydrogen bonding as an important factor in water dissolution, and they indicate that changes of water speciation are driven by the entropy and not by the volume of the system. Water in a concentration greater than 1 percent by weight also causes a marked decrease of the shear modulus of the glass.
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