Substances that dissolve in water add to the volume of the solution.
The apparent molar volume of a salt is usually less than the molar volume of the solid salt. For instance, solid NaCl has a volume of 27 cm3 per mole, but the apparent molar volume at low concentrations is only 16.6 cc/mole. In fact, some aqueous electrolytes have negative apparent molar volumes: NaOH -6.7, LiOH -6.0, and Na2CO3 -6.7 cm3/mole. This means that their solutions in a given amount of water have a smaller volume than the same amount of pure water.