Here's what happens when you lower the pH of a sodium benzoate solution, form a precipitate, and then raise the pH again:

1. Lowering the pH:

* Reaction: When you lower the pH of a solution containing sodium benzoate (NaC₇H₅O₂), you're essentially adding acid (H⁺). This acid reacts with the benzoate anion (C₇H₅O₂^-) to form benzoic acid (C₇H₆O₂):

```

C<sub>7</sub>H<sub>5</sub>O<sub>2</sub><sup>-</sup>(aq) + H<sup>+</sup>(aq) ⇌ C<sub>7</sub>H<sub>6</sub>O<sub>2</sub>(aq)

```

* Precipitation: Benzoic acid is a weak acid and is less soluble in water than its salt form, sodium benzoate. As the concentration of benzoic acid increases, it reaches its solubility limit and precipitates out of solution as a white solid.

2. Raising the pH:

* Reaction: Adding NaOH to the solution increases the pH (makes it more basic). The hydroxide ions (OH^-) from NaOH react with the benzoic acid, converting it back to benzoate ions:

```

C<sub>7</sub>H<sub>6</sub>O<sub>2</sub>(aq) + OH<sup>-</sup>(aq) ⇌ C<sub>7</sub>H<sub>5</sub>O<sub>2</sub><sup>-</sup>(aq) + H<sub>2</sub>O(l)

```

* Dissolution: As the concentration of benzoate ions increases, the benzoic acid precipitate will dissolve. This is because the equilibrium shifts back towards the formation of the more soluble benzoate ions.

In Summary:

By lowering the pH, you create an environment that favors the formation of benzoic acid, leading to precipitation. By raising the pH again, you shift the equilibrium back to the formation of benzoate ions, causing the precipitate to dissolve.