Here's why:

* Solubility: Naphthalene is a non-polar molecule, while mineral oil is also a non-polar substance. The general rule of thumb is "like dissolves like." This means that polar substances dissolve well in other polar substances, and non-polar substances dissolve well in other non-polar substances.

* Weak Interactions: While both are non-polar, the intermolecular forces between naphthalene molecules are stronger than the forces between naphthalene and mineral oil molecules. This means there's not enough attraction between them to overcome the forces holding the naphthalene crystals together.

Instead of dissolving, naphthalene crystals might:

* Suspend: They could form a suspension in mineral oil, where they remain dispersed but not truly dissolved.

* Dissolve slightly: There might be a very small amount of naphthalene that dissolves in mineral oil, but it would be negligible.

Let me know if you have any other questions!