Density measurements on several hydrous (less than or equal to 19 mole percent of H2O) silicate melts demonstrate that dissolved water has a partial molar volume ((V) over bar(H2O),) that is independent of the silicate melt composition, the total water concentration, and the speciation of water. The derived value for (V) over bar(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 derivative (V) over bar(H2O)/partial derivative T) and compressibility (partial derivative (V) over bar(H2O)/partial derivative P) 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 similar to 400 degrees C or decreasing the pressure of the melt by similar to 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.