Which is incorrect:-

Options

A. `{:("Mixture","Separated by"),((1)Na_(2)S&CuS,H_(2)O):}`
B. `{:("Mixture","Separated by"),((2)PbS & ZnS, NaCN):}`
C. `{:("Mixture","Separated by"),((3)Fe_(2)O_(3)","Al_(2)O_(3),HCl):}`
D. `{:("Mixture","Separated by"),((4)Al_(2)O_(3)","MgO,NaOH):}`

Answer

To solve the question "Which is incorrect?", we need to analyze each option provided and determine if the statements are true or false.

### Step-by-Step Solution:

1. **Option A: Na2S and CuS can be separated by H2O.**
- Both Na2S (sodium sulfide) and CuS (copper(II) sulfide) are ionic compounds.
- Na2S is more soluble in water than CuS because Na+ ions are larger than Cu2+ ions, leading to higher solubility for Na2S.
- Therefore, this statement is **correct**.

2. **Option B: PbS and ZnS can be separated using NACN as a depressant in froth flotation.**
- In froth flotation, NACN (sodium cyanide) is used as a depressant for ZnS.
- NACN forms a complex with ZnS (Na2Zn(CN)4) which prevents it from forming froth, while PbS can form froth.
- Hence, this statement is **correct**.

3. **Option C: Fe2O3 and Al2O3 can be separated by HCl.**
- Al2O3 (aluminum oxide) is soluble in NaOH, while Fe2O3 (iron(III) oxide) is insoluble in NaOH.
- They cannot be separated by HCl, as HCl does not selectively dissolve either of these oxides.
- Therefore, this statement is **incorrect**.

4. **Option D: Al2O3 is highly soluble in NaOH, while MgO is insoluble in NaOH.**
- Al2O3 is indeed soluble in NaOH, and MgO (magnesium oxide) is insoluble in NaOH.
- This statement is **correct**.

### Conclusion:
The incorrect statement is **Option C**: "Fe2O3 and Al2O3 can be separated by HCl."