Science — AI-generated practice question

Dry HCl gas does not ionise in the absence of water, so it cannot produce H⁺ ions. Since no H⁺(aq) ions are formed, it shows no acidic property and does not change the colour of dry litmus paper.

When HCl is dissolved in water, it ionises completely:

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

The H⁺ ions (hydronium ions) produced are responsible for its acidic nature, which turns blue litmus red.

Thus, water is essential for an acid to show its acidic behaviour.

Source: Chapter 2, Section 2.2.1

---

• The key concept tested is: water is necessary for ionisation of HCl.
• Examiners expect you to state that dry HCl has no H⁺ ions → no colour change, and that in solution H₃O⁺ ions form → acidic properties shown.
• Writing the equation $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$ earns a dedicated mark.
• This is directly from Activity 2.9 and the textbook statement: "The separation of H⁺ ion from HCl molecules cannot occur in the absence of water."