Here's why:

* Porous paper: This refers to filter paper, which has tiny pores that allow certain substances to pass through while blocking others.

* Size: The size of the pores in the filter paper determines which molecules can pass through. Smaller molecules can pass through, while larger molecules get trapped.

* Solubility: The solvent used in filtration can influence the separation. For example, if a molecule is soluble in the solvent, it will pass through the filter paper more easily.

Important Note: While size and solubility are key factors in filtration, the specific type of filtration you're referring to might be size-exclusion chromatography, also known as gel filtration chromatography, which uses a gel matrix instead of just paper.

Let me know if you have any further questions!