Which of the following substances is most soluble in water?

• A. BaSO_4
• B. AgCl
• C. NaNO_3
• D. PbI_2

Answer: (C)

Verified step by step guidance

1. Step 1: Understand that solubility in water depends on the nature of the compound—whether it is ionic and how well its ions interact with water molecules. Generally, ionic compounds with ions that are highly hydrated by water tend to be more soluble.
2. Step 2: Recognize that NaNO_3 (sodium nitrate) is composed of Na^+ and NO_3^- ions, both of which are highly soluble in water due to strong ion-dipole interactions with water molecules.
3. Step 3: Compare this with BaSO_4, AgCl, and PbI_2, which are all sparingly soluble salts because their lattice energies are relatively high and their ions do not interact as strongly with water, leading to low solubility.
4. Step 4: Recall that solubility rules state that most nitrates (NO_3^-) are soluble in water, while sulfates, chlorides, and iodides of heavy metals like Ba^{2+}, Ag^+, and Pb^{2+} tend to be less soluble or sparingly soluble.
5. Step 5: Conclude that among the given options, NaNO_3 is the most soluble in water because it dissociates completely and its ions are well stabilized by hydration.