Question

Determine whether each compound is soluble or insoluble. If the compound is soluble, list the ions present in solution. a. AgI b. \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) c. \(\mathrm{CoCO}_{3}\) d. \(\mathrm{K}_{3} \mathrm{PO}_{4}\)

Short answer

a. Insoluble, b. Insoluble, c. Insoluble, d. Soluble (produces \(3\mathrm{K}^{+}\) ions and \(1\mathrm{PO}_{4}^{3-}\) ion in solution).

Step-by-step solution

Apply the solubility rules to AgI

According to solubility rules, salts containing silver (Ag+) are usually insoluble, with some exceptions that do not include iodides (I-). Therefore, AgI is insoluble and does not form ions in solution.

Apply the solubility rules to \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

Compounds containing copper (Cu2+) and phosphate (PO4^3-) are generally insoluble. \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) does not dissolve in water and no ions are present in solution.

Apply the solubility rules to \(\mathrm{CoCO}_{3}\)

Most carbonates (except those of alkali metals and ammonium) are insoluble. Cobalt (II) carbonate, \(\mathrm{CoCO}_{3}\), is insoluble in water and there are no ions present in solution.

Apply the solubility rules to \(\mathrm{K}_{3} \mathrm{PO}_{4}\)

Salts containing potassium (K+) are soluble. Therefore, \(\mathrm{K}_{3} \mathrm{PO}_{4}\) is soluble and dissociates into \(3\mathrm{K}^{+}\) ions and \(1\mathrm{PO}_{4}^{3-}\) ion in solution.

Key concepts

Solubility of Compounds

When we talk about the solubility of compounds, we're looking at the ability of a substance to dissolve in a solvent, such as water. Solubility is not just a random phenomenon; it's governed by definite solubility rules which predict the outcome of mixing compounds with solvents. A compound can be classified as soluble if it dissolves to a significant extent in a specific solvent, otherwise, it is considered insoluble.

For students tackling chemistry problems, the key to predicting solubility lies in these rules. For example, most nitrate (NO₃^-) salts are soluble, while many sulfides (S^2-) are not. Understanding these patterns helps when working out which ions will be present in a solution. Remember, just because a compound is soluble, doesn't mean it'll dissolve indefinitely; each soluble substance has a maximum concentration called its saturation point. Beyond that, additional solute will not dissolve.

Solubility is crucial in fields ranging from pharmacology, where drug absorption depends on it, to environmental science, where it determines pollutant spread. Hence, a firm grasp of solubility rules is a substantial tool for any science student.

Ionic Solubility in Water

Water's polar nature makes it an excellent solvent, particularly for ionic compounds. These substances are composed of positively and negatively charged ions held together by strong electrostatic forces. When ionic compounds dissolve in water, the polar water molecules surround the separate ions, reducing the electrostatic attractions between them and allowing the ions to spread out into the solution.

This process, known as hydration, is why compounds like table salt (NaCl) and potassium iodide (KI) readily dissolve in water, separating into their respective ions. Not all ionic compounds share this solubility, as some, like silver chloride (AgCl), have ions that attract each other so strongly that water molecules cannot separate them efficiently, rendering these compounds insoluble.

It's important for students to know that solubility can be temperature dependent. Some ionic compounds might be less soluble in cold water and more soluble in hot. Moreover, not every soluble compound behaves the same way; different ionic compounds will have different solubility levels, which are precisely detailed in solubility charts that students should familiarize themselves with.

Dissociation of Ionic Compounds

In a chemical context, dissociation refers to the separation of particles in a compound, and this term is particularly important when discussing ionic compounds in water. The dissociation of ionic compounds is their split into individual ions when dissolved in water. This process is central to many reactions in aqueous solutions.

An understanding of dissociation is essential when predicting the conductivity of an ionic solution or its reactivity in chemical reactions. In an exercise where you identify the ions present in a solution, you focus on the compounds that are soluble and, therefore, dissociate. For instance, if you dissolved \(\mathrm{K}_{3} \mathrm{PO}_{4}\) in water, it would dissociate into three potassium ions (K⁺) and one phosphate ion (PO₄^3-).

It's vital for students to understand that not all dissociation is complete. Some ionic compounds partially dissociate, existing in equilibrium between the dissociated ions and the undissociated compound. Familiarity with strong and weak electrolytes can aid in predicting the extent of dissociation, which in turn has bearings on reaction rates and equilibrium positions.