Presence of Species in Dilute Aqueous Solution of Hydrochloric Acid

In a dilute aqueous solution of hydrochloric acid (HCl), several species are present. These include:

1. Hydronium ions (H3O+): The primary species in the solution are hydronium ions, which are formed when HCl molecules dissociate in water. The dissociation reaction is as follows:
HCl + H2O → H3O+ + Cl-
The HCl molecules donate a proton (H+) to water, resulting in the formation of hydronium ions.

2. Chloride ions (Cl-): When HCl molecules dissociate in water, they also release chloride ions. These ions have a negative charge and are attracted to the positive charge of the hydronium ions.

3. Un-ionized HCl: Some HCl molecules in the solution remain un-ionized, meaning they do not dissociate into hydronium and chloride ions. Instead, they exist as intact HCl molecules.

Explanation of Correct Answer:
The correct answer is option A, which states that hydronium ions (H3O+) and chloride ions (Cl-) are present in a dilute aqueous solution of hydrochloric acid. This is because HCl molecules dissociate in water to form hydronium and chloride ions. The presence of these ions makes the solution acidic.

Explanation of Incorrect Options:
Option B, which suggests the presence of hydronium ions (H3O+) and hydroxide ions (OH-), is incorrect. Hydrochloric acid is a strong acid, meaning it fully dissociates into hydronium and chloride ions but does not produce hydroxide ions. Hydroxide ions are typically present in solutions of strong bases, not strong acids.

Option C, which proposes the presence of chloride ions (Cl-) and hydroxide ions (OH-), is also incorrect for the same reason. Hydrochloric acid does not produce hydroxide ions upon dissociation.

Option D, which suggests the presence of un-ionized HCl only, is incorrect. While some HCl molecules may remain un-ionized in the solution, the primary species are hydronium and chloride ions, as explained earlier.

Overall, the correct answer is option A, as it accurately describes the species present in a dilute aqueous solution of hydrochloric acid.