Question

Determine whether each compound is soluble or insoluble. For the soluble compounds, list the ions present in solution. \(\begin{array}{ll}{\text { a. } \operatorname{AgI}} & {\text { b. } \operatorname{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}} \\ {\text { c. } \mathrm{CoCO}_{3}} & {\text { d. } \mathrm{K}_{3} \mathrm{PO}_{4}}\end{array}\)

Short answer

AgI, Cu3(PO4)2, and CoCO3 are insoluble. K3PO4 is soluble and dissociates into 3K+ and PO4^3- ions.

Step-by-step solution

Apply solubility rules for AgI

According to the solubility rules, most compounds of silver are insoluble, with some exceptions such as nitrates and acetates. Since iodides are not listed among the exceptions, AgI is insoluble.

Apply solubility rules for Cu3(PO4)2

Most phosphates are insoluble in water. Since copper (II) phosphate is not an exception to this rule, Cu3(PO4)2 is insoluble.

Apply solubility rules for CoCO3

Carbonates are generally insoluble except for those of alkali metals and ammonium. Cobalt(II) carbonate, CoCO3, being neither, is insoluble.

Apply solubility rules for K3PO4

All potassium compounds are soluble due to potassium being an alkali metal. Therefore, K3PO4 is soluble and dissociates into ions in solution: 3K+ and PO4^3-.

Key concepts

Solubility Rules

Understanding the solubility rules in chemistry is essential for predicting whether an ionic compound will dissolve in water, creating a homogeneous mixture called a solution. These rules provide a guide to determine the solubility of various compounds based on their constituent ions. As we dive into this concept, let’s start by recapping that solubility is the ability of a substance to dissolve in a solvent.

For instance, when it comes to compounds containing silver such as AgI (silver iodide), the rules suggest that most silver compounds are not water soluble, except for those combined with certain anions like nitrates or acetates. Hence, applying these rules, we consider AgI insoluble.

Key Points of Solubility Rules

• Alkali metal compounds, like those containing potassium or sodium, are generally soluble.
• Nitrates, acetates, and most perchlorates are soluble, with few exceptions.
• Carbonates, phosphates, sulfides, and oxides are often insoluble, except when paired with alkali metals or ammonium.
• Halides are usually soluble, but those of silver, lead, and mercury(II) are typically exceptions.

These are just a few solubility guidelines. Knowing these rules can greatly assist in predicting the behavior of substances in aqueous solutions and during chemical reactions.

Ionic Compounds in Solution

When ionic compounds dissolve in water, they break apart into their constituent ions in a process known as dissociation. This is a critical concept for understanding ionic compounds in solution. Soluble ionic compounds, like K₃PO₄ (potassium phosphate), dissociate to release individual ions into the solution, thereby conducting electricity due to the presence of these free-moving charged particles.

As mentioned with K₃PO₄, it is soluble, so in solution, it separates into three potassium ions (3K⁺) and one phosphate ion (PO₄^3-). The solubility of an ionic compound is not only a yes-or-no question; it's also a question of how much will dissolve, referred to as solubility product (K_sp).

Understanding Dissociation

• Not all ionic compounds dissociate completely; this is referred to as partial dissociation.
• The solubility product, K_sp, helps in quantifying the extent of solubility for sparingly soluble salts.
• Electrolytes are substances that produce ions in solution, thereby conducting an electric current.

When studying ionic solutions, it’s critical to consider the concentration of ions, which impacts both the properties of the solution and the potential for chemical reactions to occur.

Precipitation Reactions

A precipitation reaction occurs when two soluble ionic compounds are mixed together in solution and form an insoluble compound, known as the precipitate. This process is the reverse of dissolution - it’s when an ionic compound comes out of solution and forms a solid. From our examples, while K₃PO₄ is soluble, compounds like AgI, Cu₃(PO₄)₂, and CoCO₃ are insoluble and could be the products of precipitation reactions.

In practice, not all combinations of ionic solutions will create a precipitate. Such outcomes are predicted using the solubility rules. For example, if a solution of potassium iodide (KI) were combined with lead(II) nitrate (Pb(NO₃)₂), lead(II) iodide (PbI₂) would precipitate because, according to the solubility rules, iodides are soluble except with lead, silver, or mercury.

Key Aspects of Precipitation

• The formation of a precipitate can be indicative of a chemical change.
• Precipitation reactions are often used in qualitative analysis to determine the presence of certain ions in a solution.
• The net ionic equation provides a more accurate representation of the reaction by displaying only the participating ions.

Understanding these reactions is not only fundamental in analytical chemistry but also in various industries, including water treatment, where controlling the solubility of compounds is crucial.