Copper(II) nitrate (Cu(NO₃)₂) and sodium hydroxide (NaOH) react to form a precipitate because they undergo a double displacement reaction that produces an insoluble compound.

Here's a breakdown:

1. Reactants:

- Copper(II) nitrate (Cu(NO₃)₂) is a soluble ionic compound.

- Sodium hydroxide (NaOH) is also a soluble ionic compound.

2. Reaction:

When these two solutions are mixed, the copper(II) ions (Cu²⁺) from the nitrate solution react with the hydroxide ions (OH⁻) from the sodium hydroxide solution.

3. Product:

This reaction forms copper(II) hydroxide (Cu(OH)₂), a solid, blue-green precipitate.

4. Solubility:

The solubility rules state that most hydroxides are insoluble, and this applies to copper(II) hydroxide.

5. Net ionic equation:

The net ionic equation shows the key species involved:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

In summary: The reaction between copper(II) nitrate and sodium hydroxide results in the formation of insoluble copper(II) hydroxide, which precipitates out of the solution as a blue-green solid.