When sodium hydroxide (NaOH) is added to a copper sulfate (CuSO₄) solution, a chemical reaction occurs resulting in the formation of a blue precipitate. Here's a breakdown of what happens:

1. The Reaction:

* Sodium hydroxide (NaOH) is a strong base, while copper sulfate (CuSO₄) is a soluble salt. When they react, a double displacement reaction occurs.

* The sodium ions (Na⁺) from NaOH combine with the sulfate ions (SO₄²⁻) from CuSO₄ to form sodium sulfate (Na₂SO₄), which remains dissolved in the solution.

* The copper ions (Cu²⁺) from CuSO₄ combine with the hydroxide ions (OH⁻) from NaOH to form copper(II) hydroxide (Cu(OH)₂).

2. Precipitation:

* Copper(II) hydroxide (Cu(OH)₂) is insoluble in water, meaning it doesn't dissolve.

* As a result, it forms a solid precipitate, which appears as a blue gelatinous mass in the solution.

The Chemical Equation:

```

CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq)

```

Key Points:

* This reaction is a classic example of a precipitation reaction.

* The formation of the blue precipitate is a clear indication that the reaction has occurred.

* The solution will also become slightly more basic (alkaline) due to the presence of excess hydroxide ions.

Note: If you continue to add sodium hydroxide, the blue precipitate will eventually dissolve again, forming a deep blue solution containing the complex ion [Cu(OH)₄]²⁻.