Density measurements on several hydrous (</=19 mole percent of H2O) silicate melts demonstrate that dissolved water has a partial molar volume (V&cjs1171;H2O) that is independent of the silicate melt composition, the total water concentration, and the speciation of water. The derived value for V&cjs1171;H2O is 22.9 +/- 0.6 cubic centimeters per mole at 1000 degrees C and 1 bar of pressure, whereas the partial molar thermal expansivity ( partial differentialV&cjs1171;H2O/ partial differentialT) and compressibility ( partial differentialV&cjs1171;H2O/ partial differentialP) are 9.5 +/- 0.8 x 10(-3) cubic centimeters per mole per kelvin and -3.2 +/- 0.6 x 10(-4) cubic centimeters per mole per bar, respectively. The effect of 1 weight percent dissolved H2O on the density of a basaltic melt is equivalent to increasing the temperature of the melt by approximately 400 degrees C or decreasing the pressure of the melt by approximately 500 megapascals. These measurements are used to illustrate the viability of plagioclase sinking in iron-rich basaltic liquids and the dominance of compositional convection in hydrous magma chambers.