You can confirm that a substance has been converted to a different substance by showing that the substances have different properties. your observations should have revealed a difference in at least on property of sodium benzoate and benzoic acid. what property was that, and what observation revealed the difference?

The structure of sodium benzoate and benzoic acid is shown in the image.

Sodium benzoate is a sodium salt of benzoic acid.

Benzoic acid is an organic compound and weak acid means it does not dissociates in water that means it is sparingly soluble in water. Whereas the sodium benzoate is an ionic compound and dissociates completely in water to give sodium ions and benzoate ions that means it is highly soluble in water.

Thus, through solubility of benzoic acid and sodium benzote in water we can show difference between benzoic acid and sodium benzote. As, benzoic acid is sparingly soluble and sodium benzoate is highly soluble in water.