Identify the correct order of solubility in aqueous medium:

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1. **Na₂S (Sodium Sulfide):** Salts of alkali metals (Group 1) are generally soluble in water. Therefore, $Na_2S$ has the highest solubility.
2. **ZnS vs CuS:** Transition metal sulfides are generally sparingly soluble. The solubility is determined by their Solubility Product constants ($K_{sp}$).
   - **CuS:** Precipitates in Group II of qualitative analysis in the presence of dilute HCl (acidic medium). This indicates a very low $K_{sp}$ (approx $10^{-36}$), meaning it is highly insoluble [Source 235 mentions CuS has 'such a low solubility'].
   - **ZnS:** Precipitates in Group IV of qualitative analysis only in the presence of $NH_4OH$ (alkaline medium). It requires a higher concentration of sulfide ions to precipitate, indicating a comparatively higher $K_{sp}$ (approx $10^{-24}$) than CuS [Source 235 confirms ZnS precipitates in alkaline solution].
3. **Order:** Soluble ($Na_2S$) > Sparingly Soluble ($ZnS$) > Highly Insoluble ($CuS$).

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