Which of the following are present in a dilute aqueous solution of hydrochloric acid?

Options

A. `H_(3)O^(+)+Cl^(-)`
B. `H_(3)O^(+) + OH^(-)`
C. `Cl^(-) + OH^(-)`
D. Unionised HCl

Explanation

To determine which species are present in a dilute aqueous solution of hydrochloric acid (HCl), we can follow these steps:

### Step 1: Understand the Dissociation of HCl
When hydrochloric acid (HCl) is dissolved in water, it dissociates into its constituent ions. This means that the HCl molecules break apart into ions when mixed with water.

### Step 2: Write the Dissociation Reaction
The dissociation of hydrochloric acid in water can be represented by the following chemical equation:
\[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \]
This shows that one molecule of HCl produces one hydrogen ion (H⁺) and one chloride ion (Cl⁻).

### Step 3: Formation of Hydronium Ion
In an aqueous solution, the free hydrogen ions (H⁺) do not exist independently. Instead, they combine with water molecules (H₂O) to form hydronium ions (H₃O⁺). The reaction can be represented as:
\[ \text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ \]
Thus, in the solution, we have both hydronium ions and chloride ions.

### Step 4: Identify the Species Present
From the dissociation and the formation of hydronium ions, we can conclude that the following species are present in a dilute aqueous solution of hydrochloric acid:
- Hydronium ions (H₃O⁺)
- Chloride ions (Cl⁻)

### Conclusion
In a dilute aqueous solution of hydrochloric acid, the species present are:
- Hydronium ions (H₃O⁺)
- Chloride ions (Cl⁻)