Science — AI-generated practice question

Significance of H₃O⁺ and OH⁻ ions:

Acids do not show acidic properties in the absence of water. When dissolved in water, acids produce hydronium ions (H₃O⁺), as shown below:

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

H⁺ ions cannot exist alone; they combine with water to form H₃O⁺. The acidic nature of a substance is due to the formation of H⁺(aq)/H₃O⁺ ions in solution.

Similarly, bases dissolved in water release hydroxide ions:

$$\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}$$

The basic nature of a substance is due to the formation of OH⁻(aq) ions.

Neutralisation at the ionic level:

Since every acid supplies H⁺(aq) and every base supplies OH⁻(aq), the neutralisation reaction can be written as:

$$\text{HX} + \text{MOH} \rightarrow \text{MX} + \text{H}_2\text{O}$$

At the ionic level, this reduces to:

$$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$$

The H⁺ and OH⁻ ions combine to form neutral water, which is why acids and bases cancel each other's effect. The remaining ions (e.g., Na⁺ and Cl⁻) form the salt.

Source: Chapter 2, Section 2.2 / 2.2.1

---

• Examiners expect the key point that H⁺ cannot exist alone — it must be written as H⁺(aq) or H₃O⁺.
• The ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) is the core of this answer and must be written.
• Show at least one equation for acid dissociation and one for base dissociation to earn full marks.
• The significance is that all neutralisations are essentially the same reaction at the ionic level regardless of which specific acid or base is used.