Science — AI-generated practice question

A hydronium ion (H₃O⁺) is formed when a hydrogen ion (H⁺) released by an acid combines with a water molecule. Since H⁺ ions cannot exist freely in solution, they attach to water to form hydronium ions.

Equation for its formation:

$$\text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+$$

Source: Acids, Bases and Salts, Chapter 2

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• Examiners expect the definition (what it is) and the equation — both are needed for full 2 marks.
• Key point: H⁺ ions do not exist independently in aqueous solution; they always associate with water to form H₃O⁺. Mentioning this shows conceptual clarity.
• The term "hydronium ion" and its formula H₃O⁺ must be written correctly.
• The equation must be balanced with correct formulae.