Science — AI-generated practice question

H⁺ ions are highly reactive and cannot exist freely in solution. In the presence of water, H⁺ ions combine with water molecules to form hydronium ions (H₃O⁺):

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

So HCl in water dissociates as:

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

In the absence of water, HCl exists as a covalent gas (dry HCl) and does not dissociate into ions. Therefore, dry HCl does not show acidic properties — it does not change the colour of dry litmus paper. Acidic behaviour requires water for ionisation.

Source: Chapter 2, Section 2.2.1

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• The key distinction examiners want: H⁺ always combines with H₂O to form H₃O⁺ — it never exists alone.
• Both equations must be written; forgetting them loses marks.
• The contrast with dry HCl (no ions → no acidity) is the third point — directly tested in Activity 2.9 of the textbook.
• Write H⁺(aq) or H₃O⁺ — both are acceptable; the textbook uses both.