Results of ultrasonic absorption measurements of Ca(NO3)2·4H2O melts and of 1:4.3, 1:4.7, and 1:5.0 Ca(NO3)2-H2O mixtures are presented. The temperature range 25-75° and the frequency range of 3-195 MHz have been investigated. A relaxation process at 20 ± 5 MHz for Ca(NO3)2·4H2O, shifting to higher frequencies by addition of water, is reported. This relaxation tends to disappear by increasing the temperature. The volume viscosities are calculated in the frequency region below and above the observed relaxation region. The conclusion is drawn that two overlapping sources of the volume viscosities exist. At frequencies below the relaxation region, structural and nonstructural molecular rearrangements contribute to the excess sound absorption over the classical Stokes value. The energy barrier of this over-all process is larger than the one for shear viscous flow as indicated by the decrease of the ratio ηv/ηs, with temperature. At frequencies larger than the observed relaxation region only the structural contributions to the sound absorption remain. The barrier of energy for the compression volume viscosity ηv′ is the same as the one for the shear viscosity ηs. This indicates that all of the possible translation molecular rearrangements have been frozen out after the relaxation except the ones with a barrier of translational energy equal to the viscous flow.
