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

• A. Ag^+
• B. CO_3^{2-}
• C. Pb^{2+}
• D. Na^+

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 alkali metal ions (such as Na^+, K^+, Li^+) and the ammonium ion (NH_4^+) generally form soluble compounds with almost all anions.
3. Examine each ion given: Ag^+ often forms insoluble compounds (like AgCl), CO_3^{2-} (carbonate) usually forms insoluble salts except with alkali metals and NH_4^+, and Pb^{2+} forms many insoluble compounds.
4. Recognize that Na^+ is an alkali metal ion, and according to solubility rules, all sodium salts are soluble in water regardless of the anion.
5. Conclude that among the ions listed, Na^+ always forms soluble ionic compounds in water.