Predict whether Cu(NO3)2 should be soluble or insoluble in water and explain why. Select one:
a. Cu(NO3)2 is soluble because Cu-containing compounds are always soluble.
b. Cu(NO3)2 is insoluble because Cu-containing compounds are always insoluble.
c. Cu(NO3)2 is soluble because NO3-containing compounds are always soluble.
d. Cu(NO3)2 is insoluble because NO3-containing compounds are always insoluble.
The correct answer and explanation is :
The correct answer is:
c. Cu(NO3)2 is soluble because NO3-containing compounds are always soluble.
Explanation:
Copper(II) nitrate, Cu(NO₃)₂, is soluble in water because it contains the nitrate (NO₃⁻) ion. In general, solubility rules help predict whether an ionic compound will dissolve in water. One of the key solubility rules states that all nitrates (NO₃⁻) are soluble in water, regardless of which metal cation they are paired with.
1. Nitrate Ion (NO₃⁻) and Its Solubility Rule
The nitrate ion is a highly soluble anion due to its ability to form strong ion-dipole interactions with water molecules. The presence of multiple oxygen atoms allows it to easily stabilize in an aqueous solution, leading to high solubility for all nitrate salts.
2. Copper(II) Ion (Cu²⁺) and Solubility
Copper compounds can sometimes be insoluble depending on the anion they are paired with (e.g., Cu(OH)₂ and CuS are insoluble). However, when copper(II) is bonded to nitrate, the strong solubility rule of nitrate overrides any potential insolubility of copper(II), making Cu(NO₃)₂ soluble in water.
3. General Solubility Trend of Nitrate Salts
- Soluble: NaNO₃, KNO₃, Ca(NO₃)₂, Al(NO₃)₃, Cu(NO₃)₂
- No exceptions: Unlike other anions such as sulfate (SO₄²⁻), which have exceptions (e.g., PbSO₄ and BaSO₄ are insoluble), all nitrate salts are soluble without exception.
Conclusion:
Since Cu(NO₃)₂ follows the nitrate solubility rule, it dissolves completely in water, forming Cu²⁺ and NO₃⁻ ions in solution. Therefore, option (c) is correct.