Why will filtering not separate solute from a solution using the sizes of particles?

Answer 1

A solute is dissolved in the solution, so if for example you are talking about sugar and water, the white sugar is invisible and the water is clear. So when you filter the water/sugar solution everything filters through the filter paper. If the solution is saturated, this means that the water has dissolved as much as possible so there will be some sugar remaining in the solution. If this is to be filtered then the undissolved sugar will remain in the filter paper, separating the sugar/water solution from the undissolved sugar.

Answer 2

You can use filtering when there are actual particles large enough to be caught in the filter's gaps. So, for example, if you have water mixed with sand and you pass the whole mixture through a filter, because sand particles are larger than the filter's gaps, they will get caught in it, and only water will pass. This happens because the sand particles are relatively large bodies that don't actually make part of the liquid that is water. However, when you dissolve something in water, salt for example, the individual atoms that make up the salt bond to the water molecules; thus, the salt becomes part of the liquid water itself. Since the salt is bonded to the water at a molecular level (note: this is not a chemical bond, such as those formed and broken in chemical reactions), large particles that could get caught in a filter simply don't form, so filtering won't work.