The particle responsible for acid properties in water is the hydronium ion (H₃O⁺).

Here's why:

* Acids donate protons (H⁺): Acids are substances that donate protons (hydrogen ions, H⁺) when dissolved in water.

* Hydronium formation: When an acid dissolves in water, the proton (H⁺) from the acid combines with a water molecule (H₂O) to form a hydronium ion (H₃O⁺).

* Increased hydronium concentration: The higher the concentration of hydronium ions in a solution, the more acidic the solution.

Example:

Hydrochloric acid (HCl) is a strong acid. When dissolved in water, it donates a proton to a water molecule, forming hydronium ions:

HCl + H₂O → H₃O⁺ + Cl⁻