Mixtures and fluid flow
A mixture is sorted by whether you can see its parts, and a fluid is described by its density and its viscosity.
A worked example
A block with a mass of 180 g filling 90 cm3 has a density of 180 / 90 = 2 g/cm3, so it sinks in water at about 1 g/cm3.
The mistake people actually make
Density and viscosity are not the same property: cooking oil is less dense than water yet pours far more slowly. In a solution the solvent is whichever substance is present in the larger amount.