The first experimental step after weighing and combining your salt/sand mixture is adding water to your mixture and stirring. After you have stirred for a few minutes, where are the SiO2 and NaCl, respectively?

NaCl dissolves into the water and dissociates into Na⁺ and Cl⁻ ions, present homogeneously throughout the solvent

SiO₂ settles down at the bottom of the vessel in the form of coarse particles

Explanation:

NaCl is a strong electrolyte that is, it dissociates readily in an aqueous solution. So when the water is added in the mixture, NaCl dissolves readily and is present uniformly in the solvent as Na⁺ and Cl⁻ ions.

SiO₂ has a strong ionic bond and is not dissociated by water, so it is present in the form of coarse particles at the bottom of the vessel.