Here's why:

* Ionic Compounds: Copper(II) nitrate is an ionic compound, meaning it's formed by the electrostatic attraction between positively charged copper ions (Cu²⁺) and negatively charged nitrate ions (NO₃⁻).

* Polarity: Water is a polar solvent, meaning it has a positive and negative end due to the uneven sharing of electrons.

* Dissolving: When copper(II) nitrate is added to water, the polar water molecules surround the ions, weakening the electrostatic forces holding the compound together. This allows the ions to separate and dissolve in the water.

Note: The solubility of copper(II) nitrate in water increases with temperature.