To determine which of the following substances is an Arrhenius acid, we need to analyze each one based on the Arrhenius definition of acids and bases. According to Arrhenius, an acid is a substance that increases the concentration of hydrogen ions (H⁺) when dissolved in water, and a base is a substance that increases the concentration of hydroxide ions (OH⁻) when dissolved in water.
Let's examine each substance:
1. [tex]\( BF_3 \)[/tex] (Boron trifluoride):
- Boron trifluoride is not an acid by the Arrhenius definition. It does not release H⁺ ions when dissolved in water. It is more commonly known as a Lewis acid, as it can accept electron pairs, but that is not relevant to the Arrhenius definition.
2. HCN (Hydrogen cyanide):
- Hydrogen cyanide, when dissolved in water, can dissociate to release H⁺ ions:
[tex]\[
HCN \rightarrow H^+ + CN^-
\][/tex]
- This means that HCN is an Arrhenius acid as it increases the concentration of hydrogen ions in solution.
3. [tex]\( NH_3 \)[/tex] (Ammonia):
- Ammonia is a base under the Arrhenius definition. It reacts with water to produce hydroxide ions:
[tex]\[
NH_3 + H_2O \rightarrow NH_4^+ + OH^-
\][/tex]
- Therefore, [tex]\( NH_3 \)[/tex] is not an acid but a base.
4. [tex]\( Mg(OH)_2 \)[/tex] (Magnesium hydroxide):
- Magnesium hydroxide is a base since it produces hydroxide ions when dissolved in water:
[tex]\[
Mg(OH)_2 \rightarrow Mg^{2+} + 2OH^-
\][/tex]
- Thus, [tex]\( Mg(OH)_2 \)[/tex] is also not an acid but a base.
Based on this analysis, HCN (Hydrogen cyanide) is the only substance among the given options that qualifies as an Arrhenius acid. Therefore, the correct answer is:
HCN
