Final answer:
option 2,HCl (hydrochloric acid) is an Arrhenius acid because it increases the concentration of hydronium ions when dissolved in water.
Step-by-step explanation:
The compound that is an Arrhenius acid from the list provided is HCl (hydrochloric acid). An Arrhenius acid is defined as a substance that, when dissolved in water, increases the concentration of hydrogen ions (H+), which in the presence of water form hydronium ions (H3O+). The dissociation of HCl in water can be represented by the following chemical equation:
HCl(g) + H2O(l) → H3O+(aq) + Cl−(aq)
This shows that when HCl is dissolved in water, it produces hydronium ions, indicating its property as an Arrhenius acid. On the other hand, CaO (calcium oxide), K2O (potassium oxide), and NH3 (ammonia) do not produce hydrogen ions in solution; in fact, NH3 is a classic example of an Arrhenius base as it produces hydroxide ions (OH−) in solution.
An Arrhenius acid is a compound that increases the concentration of H+ ions that are present when added to water. These H+ ions form the hydronium ion (H3O+) when they combine with water molecules. For the compounds given in the options, only HCl is an Arrhenius acid because it can dissociate in water to yield H+ ions.