Which pair of compounds is soluble in water?

• A. AgCl and PbSO_4
• B. CuS and Fe(OH)_3
• C. NaNO_3 and K_2SO_4
• D. BaSO_4 and CaCO_3

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of solubility rules in water, which help predict whether an ionic compound will dissolve. Generally, compounds containing alkali metal ions (like Na⁺, K⁺) and nitrate ions (NO₃⁻) are soluble in water.
2. Step 2: Analyze each pair of compounds by applying common solubility rules: for example, most chlorides (Cl⁻) are soluble except those of Ag⁺, Pb²⁺, and Hg₂²⁺; most sulfates (SO₄²⁻) are soluble except those of Ba²⁺, Pb²⁺, and Ca²⁺; most hydroxides (OH⁻) are insoluble except those of alkali metals and some alkaline earth metals; most sulfides (S²⁻) and carbonates (CO₃²⁻) are generally insoluble except with alkali metals and ammonium.
3. Step 3: Evaluate the given pairs: AgCl and PbSO₄ both contain ions known to form insoluble compounds (AgCl is insoluble, PbSO₄ is sparingly soluble), CuS and Fe(OH)₃ are generally insoluble, BaSO₄ and CaCO₃ are also mostly insoluble in water.
4. Step 4: Identify that NaNO₃ and K₂SO₄ contain alkali metal ions (Na⁺, K⁺) and nitrate or sulfate ions, which according to solubility rules, are soluble in water.
5. Step 5: Conclude that the pair NaNO₃ and K₂SO₄ is soluble in water based on the solubility rules, while the other pairs are not.