which of the following substances is an arrhenius acid?\no bf₃\no hcn\no nh₃\no mg(oh)₂

Answer:

B. HCN

Explanation:

Step1: Recall Arrhenius acid definition

An Arrhenius acid is a substance that dissociates in water to produce H⁺ ions.

Step2: Analyze each option

• BF₃ is a Lewis - acid, not an Arrhenius acid as it does not dissociate to give H⁺ in water.
• HCN dissociates in water as HCN ⇌ H⁺+CN⁻, so it is an Arrhenius acid.
• NH₃ is a base as it accepts H⁺ in water (NH₃ + H₂O ⇌ NH₄⁺+OH⁻), not an acid.
• Mg(OH)₂ is a base as it dissociates in water to give OH⁻ ions (Mg(OH)₂ ⇌ Mg²⁺ + 2OH⁻), not an acid.