Which of the comparison Zn, Cd, Hg is/are incorrect?
(I) `ZnCl_(2)` is ionic whereas ` CdCl_(2) and HgCl_(2)` is covalent
(II) Zn and Cd dissolves in dilute acid HCl liberating `H_(2)` but Hg can not
(III) Zn and cd forming with ppt. of `Zn(OH)_(2) and Cd(OH)_(2)` but Hg forms coloured ppt. of `Hg(OH)_(2)`.
(IV) All form `A_(2)^(2+)` type ion

Options

A. Only III
B. I, III, IV
C. I and IV
D. All of these

Answer

To determine which of the comparisons between Zn, Cd, and Hg are incorrect, we will analyze each statement one by one.

### Step 1: Analyze Statement I
**Statement I:** ZnCl₂ is ionic whereas CdCl₂ and HgCl₂ are covalent.

- **Zinc Chloride (ZnCl₂):** This compound is indeed ionic. It dissociates into Zn²⁺ and Cl⁻ ions in solution.
- **Cadmium Chloride (CdCl₂):** This compound is also ionic. It dissociates into Cd²⁺ and Cl⁻ ions in solution.
- **Mercury Chloride (HgCl₂):** This compound is covalent in nature, as it does not dissociate into ions in solution.

**Conclusion for Statement I:** The statement is incorrect because both ZnCl₂ and CdCl₂ are ionic.

### Step 2: Analyze Statement II
**Statement II:** Zn and Cd dissolve in dilute HCl liberating H₂, but Hg cannot.

- **Zinc (Zn):** Reacts with dilute HCl to liberate H₂ gas.
- **Cadmium (Cd):** Also reacts with dilute HCl to liberate H₂ gas.
- **Mercury (Hg):** Does not react with dilute HCl to liberate H₂ gas due to its lower electropositivity.

**Conclusion for Statement II:** The statement is correct.

### Step 3: Analyze Statement III
**Statement III:** Zn and Cd form precipitates of Zn(OH)₂ and Cd(OH)₂, but Hg forms a colored precipitate of Hg(OH)₂.

- **Zinc Hydroxide (Zn(OH)₂):** Forms a white precipitate.
- **Cadmium Hydroxide (Cd(OH)₂):** Also forms a white precipitate.
- **Mercury Hydroxide (Hg(OH)₂):** Forms a colored precipitate (often a white or grayish color).

**Conclusion for Statement III:** The statement is correct.

### Step 4: Analyze Statement IV
**Statement IV:** All form A₂²⁺ type ions.

- **Zinc (Zn):** Exists as Zn²⁺.
- **Cadmium (Cd):** Exists as Cd²⁺.
- **Mercury (Hg):** Exists as Hg₂²⁺ (dimeric form) or Hg²⁺.

**Conclusion for Statement IV:** The statement is incorrect because Zn and Cd do not form A₂²⁺ type ions; they exist as A²⁺.

### Final Conclusion
The incorrect statements are:
- Statement I: Incorrect (CdCl₂ is ionic)
- Statement IV: Incorrect (Zn and Cd do not form A₂²⁺ type ions)

### Summary of Incorrect Statements
- **Incorrect Statements:** I and IV