When an acid reacts with water, it produces hydronium ions (H₃O⁺). This is because acids are proton (H⁺) donors.

Here's a simplified explanation:

* Acid: An acid contains hydrogen ions (H⁺) that are readily released.

* Water: Water molecules (H₂O) can accept these hydrogen ions.

* Reaction: When an acid is added to water, the hydrogen ions from the acid bond with water molecules to form hydronium ions (H₃O⁺).

Example:

* Hydrochloric acid (HCl) in water:

* HCl → H⁺ + Cl⁻

* H⁺ + H₂O → H₃O⁺

The presence of hydronium ions is what makes the solution acidic. The more hydronium ions present, the stronger the acid.