* Water is polar: Water molecules have a bent shape with a slightly negative charge on the oxygen side and a slightly positive charge on the hydrogen side. This polarity allows water molecules to form strong hydrogen bonds with each other.
* Cyclopentane is nonpolar: Cyclopentane consists of a ring of five carbon atoms with hydrogen atoms attached. The carbon-hydrogen bonds are almost nonpolar, meaning there is very little difference in electronegativity between the carbon and hydrogen atoms. This makes cyclopentane a nonpolar molecule.
* "Like dissolves like": Polar substances dissolve well in other polar substances, and nonpolar substances dissolve well in other nonpolar substances. Since cyclopentane is nonpolar and water is polar, they are not attracted to each other.
In summary: The strong hydrogen bonding between water molecules prevents cyclopentane from breaking into the network. This leads to cyclopentane's insolubility in water.
