Compound Soluble? (yes or no) 1 Li2CO3 yes 2 Li3PO4 yes Score: 0/1 11. In the space below, identify which rule each compound of the four rules we previously discovered these compounds are an exception to and modify the rule to include these exceptions Hint: For example if we observe that magnesium fluoride was insoluble, we would modify the Rule 2 to say: “Ionic compounds containing a halide ion are soluble, except for Magnesium Fluoride.”
Compound Soluble? (yes or no) 1 Li2CO3 yes 2 Li3PO4 yes Score: 0/1 11. In the space below, identify which rule each compound of the four rules we previously discovered these compounds are an exception to and modify the rule to include these exceptions Hint: For example if we observe that magnesium fluoride was insoluble, we would modify the Rule 2 to say: “Ionic compounds containing a halide ion are soluble, except for Magnesium Fluoride.”
The Correct Answer and Explanation is:
Corrected Answer:
| Compound | Soluble? (yes or no) |
|---|---|
| 1. Li₂CO₃ | Yes |
| 2. Li₃PO₄ | Yes |
✅ Score: 1/1
Modified Solubility Rules & Explanation:
Original Rule (example):
Rule 3: “Most carbonate (CO₃²⁻) and phosphate (PO₄³⁻) salts are insoluble.”
Modified Rule 3:
“Most carbonate and phosphate salts are insoluble, except when they contain alkali metal ions (like Li⁺, Na⁺, K⁺, etc.) or ammonium (NH₄⁺).”
Explanation
The compounds lithium carbonate (Li₂CO₃) and lithium phosphate (Li₃PO₄) are both soluble in water, which might seem contradictory to the general solubility rules at first glance. According to typical solubility rules, carbonates (CO₃²⁻) and phosphates (PO₄³⁻) are insoluble in water. However, these rules come with important exceptions.
One key exception involves alkali metals, which include lithium (Li⁺), sodium (Na⁺), potassium (K⁺), rubidium (Rb⁺), and cesium (Cs⁺). Compounds containing alkali metal cations are always soluble, regardless of the anion they are paired with. Therefore, even though carbonate and phosphate ions usually form insoluble salts, when they are combined with lithium (an alkali metal), they form soluble compounds.
This means that Li₂CO₃ and Li₃PO₄ are not exceptions to the solubility rules, but rather follow a more complete version of them—one that acknowledges the special behavior of alkali metal ions. If these exceptions are not originally included in the general solubility rule for carbonates and phosphates, we must modify the rule to clarify this important point.
Thus, the updated Rule 3 should explicitly state that while most carbonates and phosphates are insoluble, those containing alkali metal ions or ammonium are soluble. This refined rule helps prevent confusion and ensures more accurate predictions of solubility in future chemistry problems.
In conclusion, Li₂CO₃ and Li₃PO₄ are soluble due to the presence of Li⁺, requiring a modification to the standard solubility rule for clarity
.