Final answer:
HBr, hydrogen bromide, is the correct choice as it dissociates in water to produce hydrogen ions (H+), making it an Arrhenius acid. Option 2 is correct.
Step-by-step explanation:
The question is which substance is an Arrhenius acid. An Arrhenius acid is a substance that produces hydrogen ions (H+) when dissolved in water. Given the list of compounds, we need to identify which one behaves as an Arrhenius acid:
LiF is lithium fluoride and is neither an Arrhenius acid nor base.
HBr, hydrogen bromide, dissociates in water to produce H+ ions, therefore HBr is the Arrhenius acid here.
Mg(OH)2 is magnesium hydroxide, an Arrhenius base as it produces OH− ions.
CH3CHO is acetaldehyde and does not produce H+ or OH− ions in solution; therefore, it is neither an Arrhenius acid nor base.
Therefore, the compound HBr is the Arrhenius acid among the ones listed.
An Arrhenius acid is a substance that produces hydrogen ions (H+) in aqueous solution. In the given options, the only substance that fits this definition is HBr (hydrobromic acid). Therefore, the answer is HBr.