Video Transcript
Which of the following carbonate salts has the highest solubility in water? (A) MgCO3, (B) K2CO3, (C) FeCO3, (D) CuCO3, or (E) ZnCO3.
All of the given answer choices are carbonate salts, which means the anion present in all of these ionic compounds is the carbonate ion. There are many possible tests used in chemistry to detect the presence of different ions in salts and solutions. Testing the solubility of a compound is one example. Solubility can be defined as the maximum amount of solute that can dissolve in a given amount of solvent at a given temperature. The solutes in this question are the possible carbonate salts in the answer choices, and the solvent is water.
Solubility in water ranges for completely soluble to completely insoluble. When a substance such as a salt is soluble in water, this means it dissolves and disperses throughout the water generating an aqueous salt solution. When a salt is insoluble in water, the salt does not dissolve. Instead, an insoluble salt often settles to the bottom of a container, and when formed in reactions, insoluble salts are referred to as precipitates. Using this information, we must determine which of these five carbonate salts when mixed with water will have the highest ability to dissolve.
We can reference the water solubility rules to do so. This set of rules addresses the ability to dissolve in water for many different ions. But for this question, we will focus only on the rules involving the carbonate ion. Compounds containing the carbonate anion are generally insoluble in water and will therefore not dissolve. This rule has a few exceptions. Carbonate compounds also containing alkaline metal ions such as lithium, sodium, potassium, or rubidium ions would be soluble in water as would any carbonate compounds containing the ammonium ion. So the salt with the higher solubility in water would likely contain one of these cations.
If we look at our answer choices, we can see that answer choice (B), potassium carbonate, contains potassium cations, which means this compound is an exception to the insolubility of carbonate salts. Potassium carbonate in water dissolves, and each unit dissociates into its constituent ions. Therefore, the carbonate salt that has the highest solubility in water would be answer choice (B), K2CO3.
