Why filtering cannot be used to separate a solvent from a solute?

Answer 1

because the solute has already dissolved in the solvent so when filtering the solvent and solute go together...

Filtering relies on the difference in sizes of particles of different substances. Solutes and solvents are commingled at the molecular level. There are methods of separating the two via chemical affinity that are sometimes referred to as "filtering" such as membrane separations, but they are not true filters.

There are some very limited examples of filters that actually filter based on molecular sizes. Usually these are for filtering out rather large molecules or where the molecules can be made to cluster together to effectively form a larger particle. Cold filtering of beer is such an example where the cold promotes clustering of protein molecules (solute) which can then be filtered from the rest of the beer (solvent).

Answer 2

Filtering is ineffective in separating a solvent from a solute because the solute particles are dissolved and cannot be physically filtered out. The solvent and solute form a homogeneous mixture at a molecular level, making separation by filtration impossible. Other separation techniques like evaporation or distillation are more appropriate for separating a solvent from a solute.