Final answer:
Vinegar is an acid because it contains acetic acid (CH3COOH). Sodium hydroxide is a strong base, not an acid, and ammonia is a weak base that partially dissociates in water.
Step-by-step explanation:
The student asked which out of the given choices is an acid. Among ammonia (NH3), vinegar, NaOH (sodium hydroxide), and water, the substance that is an acid is vinegar. This is because vinegar contains acetic acid (CH3COOH), which is a weak organic acid. Sodium hydroxide (NaOH), also mentioned in the choices, is a strong base and not an acid. It contains a 1:1:1 ratio of sodium (Na), oxygen (O), and hydrogen (H).
To know if other substances are acidic or basic in nature, understanding their reactions in water can be helpful. Ammonia, for example, is a weak base and only a small percentage dissociate into ammonium ions (NH4+) and hydroxide ions (OH-) in water. On the contrary, acetic acid (in vinegar) reacts with water to produce hydronium ions (H3O+) and acetate ions (CH3COO-), which is characteristic of an acid.