We are basically looking at why water soluble solids dissolve faster and more thoroughly in hot water versus cold water. Hot water has more kinetic energy associated with the solvent molecules. This increase in kinetic energy increases molecular motion at the atomic level, giving rise to increased motion within molecules and between molecules. This increase in motion allows the solvent molecules to more effectively contact the solid particles. This increased contact between solvent and solid molecules allows more of the solid to dissolve in the water quicker than cold water where there is less kinetic energy and therefore less contact.