HI is an Arrhenius acid because
A) it is a polar molecule.
B) it can dissolve in water.
C) it produces hydronium ions in solution.
D) it produces hydroxide ions in solution.
The correct answer and explanation is :
The correct answer is C) it produces hydronium ions in solution.
Explanation:
An Arrhenius acid is defined as a substance that increases the concentration of H⁺ ions (protons) in aqueous solution. When hydrogen ions are released in water, they quickly associate with water molecules to form hydronium ions (H₃O⁺).
Hydroiodic acid (HI) is a classic example of an Arrhenius acid. When HI is dissolved in water, it dissociates completely into hydrogen ions (H⁺) and iodide ions (I⁻):
[
\text{HI} \rightarrow \text{H}^+ + \text{I}^-
]
The free hydrogen ions (H⁺) immediately combine with water molecules to form hydronium ions:
[
\text{H}^+ + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+
]
Thus, the presence of hydronium ions (H₃O⁺) is a direct result of the behavior of an Arrhenius acid like HI in water.
Let’s briefly address the other options:
- A) it is a polar molecule: While HI is indeed polar due to the difference in electronegativity between hydrogen and iodine, polarity alone does not make a molecule an Arrhenius acid. Many polar molecules are not acids.
- B) it can dissolve in water: Solubility in water is important for many acids, but simply dissolving is not sufficient. A substance must also release H⁺ ions to be considered an Arrhenius acid.
- D) it produces hydroxide ions in solution: This describes a base, not an acid, according to Arrhenius theory. Bases produce OH⁻ (hydroxide ions), not H₃O⁺.
In conclusion, HI is classified as an Arrhenius acid because, when dissolved in water, it produces hydronium ions, fulfilling the definition of an Arrhenius acid. Its behavior in aqueous solutions — full dissociation and proton donation — highlights its strength as an acid as well.