Which of the following ions always form soluble ionic compounds in water?

• A. CO_3^{2-}
• B. Na^+
• C. Cl^-
• D. NO_3^-

Answer: (D)

Verified step by step guidance

1. Understand the concept of solubility rules in water, which help predict whether an ionic compound will dissolve (be soluble) or not (be insoluble).
2. Recall that certain ions, such as alkali metal cations (e.g., Na^+) and nitrate anions (NO_3^-), generally form soluble compounds with almost all other ions.
3. Recognize that carbonate ions (CO_3^{2-}) typically form insoluble compounds except when paired with alkali metal cations or ammonium ions.
4. Note that chloride ions (Cl^-) usually form soluble compounds, but there are exceptions with some heavy metal cations (like Ag^+, Pb^{2+}, Hg_2^{2+}) where the compounds are insoluble.
5. Conclude that Na^+ and NO_3^- ions always form soluble ionic compounds in water, while CO_3^{2-} and Cl^- do not always guarantee solubility.