To determine which substance is incorrectly classified, let's systematically classify each one correctly based on its chemical properties.
1. HI (aq) - acid
- HI (hydroiodic acid) is a strong acid that completely dissociates in water.
- [tex]\( \text{HI} \rightarrow \text{H}^+ + \text{I}^- \)[/tex]
- It releases hydrogen ions (H⁺) when dissolved in water, hence it is correctly classified as an acid.
2. Na₂CO₃ - amphoteric
- Sodium carbonate is a salt formed from the reaction of a strong base (NaOH) and a weak acid (H₂CO₃).
- [tex]\( \text{2NaOH} + \text{H}_2\text{CO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{2H}_2\text{O} \)[/tex]
- Sodium carbonate is a basic salt due to the presence of the carbonate ion ([tex]\(\text{CO}_3^{2-}\)[/tex]) which can react with water to form [tex]\(\text{OH}^-\)[/tex]:
- [tex]\( \text{CO}_3^{2-} + \text{H}_2\text{O} \rightarrow \text{HCO}_3^- + \text{OH}^- \)[/tex]
- Therefore, it is not amphoteric; it is a basic salt. This classification is incorrect.
3. HS⁻ - amphoteric
- Hydrogen sulfide ion ([tex]\(\text{HS}^-\)[/tex]) can act both as an acid and as a base, meaning it can donate or accept a proton.
- As an acid: [tex]\( \text{HS}^- + \text{OH}^- \rightarrow \text{S}^{2-} + \text{H}_2\text{O} \)[/tex]
- As a base: [tex]\( \text{HS}^- + \text{H}^+ \rightarrow \text{H}_2\text{S} \)[/tex]
- Therefore, [tex]\(\text{HS}^-\)[/tex] is correctly classified as amphoteric.
4. NH₄OH - base
- Ammonium hydroxide ([tex]\(\text{NH}_4\text{OH}\)[/tex]) is actually a solution of ammonia in water. In aqueous solution, it largely exists as ammonium ([tex]\(\text{NH}_4^+\)[/tex]) and hydroxide ions ([tex]\(\text{OH}^-\)[/tex]).
- [tex]\( \text{NH}_4\text{OH} \leftrightharpoons \text{NH}_4^+ + \text{OH}^- \)[/tex]
- It is a weak base, not an acid or salt. This classification as a base is correct.
5. KBr - salt
- Potassium bromide (KBr) is a salt that results from the neutralization reaction between the strong base potassium hydroxide (KOH) and the strong acid hydrobromic acid (HBr).
- [tex]\( \text{KOH} + \text{HBr} \rightarrow \text{KBr} + \text{H}_2\text{O} \)[/tex]
- KBr dissociates completely in water into potassium ([tex]\(\text{K}^+\)[/tex]) and bromide ions ([tex]\(\text{Br}^-\)[/tex]). Thus, it is correctly classified as a salt.
After reviewing each of the classifications, the incorrect classification is Na₂CO₃ which should be classified as a basic salt rather than amphoteric.
Therefore, the substance incorrectly classified is Na₂CO₃.
