Question

Indicate whether aqueous solutions of the following substances contain ions. If so, write formulas for the ions in solution. (a) \(\mathrm{I}_{2}\) (b) KI (c) \(\mathrm{CH}_{3} \mathrm{OH}\)

Short answer

(a) \(\mathrm{I}_{2}\) does not form ions in solution. (b) KI forms ions in solution: \(K^{+}\) and \(I^{-}\). (c) \(\mathrm{CH}_{3} \mathrm{OH}\) does not form ions in solution.

Step-by-step solution

Separate solvent and solute

The solvent in all solutions is water, which is a very good solvent for many substances because it can solvate a variety of different kinds of particles.

Decide which substances are soluble

We know the three substances in question: \(\mathrm{I}_{2}\), KI, and \(\mathrm{CH}_{3} \mathrm{OH}\). Nonelectrolytes like \(\mathrm{I}_{2}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) are covalent, which will not create ions in solution because they do not dissociate. KI is an ionic compound which will dissociate into ions in solution.

Write formulas for the ions

As we established that KI dissociates into ions, we can write the formulas: \(K^{+}\) and \(I^{-}\).

Summarize the results

In conclusion, \(\mathrm{I}_{2}\) and \(\mathrm{CH}_{3} \mathrm{OH}\) do not form ions in solution because they are covalent compounds and do not dissociate. KI, on the other hand, is ionic and does dissociate into ions (specifically \(K^{+}\) and \(I^{-}\)) in solution.

Key concepts

Solubility in Chemistry

When we talk about solubility in chemistry, we are referring to the ability of a substance, known as the solute, to dissolve in a solvent and form a homogeneous mixture called a solution. Water is a common solvent, often referred to as the 'universal solvent' due to its unique ability to dissolve a wide range of substances. Solubility is a critical concept because it dictates how substances interact in aqueous environments, which is particularly important in biological systems, environmental sciences, and many industrial applications.

Substances can be categorized based on their solubility behavior into three main types:

• Non-electrolytes: Such as \textsf{I}\(_2\) (iodine), do not dissolve to form ions in solution and therefore do not conduct electricity. These are typically covalent compounds which do not dissociate into ions.
• Electrolytes: Substances that dissolve to form ions, allowing the solution to conduct electricity. These can be ionic compounds like KI (potassium iodide). They dissociate to form positive and negative ions in solution.
• Nonelectrolytes: Like \textsf{CH}\(_3\)OH (methanol), dissolve in water but remain as whole molecules and do not form ions.

Solubility is influenced by temperature, pressure, and the nature of the solute and the solvent. Some substances like KI are highly soluble in water and readily form ions, which is evidenced by their ability to conduct an electrical current when dissolved.

Ionic and Covalent Compounds

To understand the behavior of substances in water, we must first talk about ionic and covalent compounds. The distinction between these two types of compounds is based on the kind of bonds holding the atoms together.

• Ionic compounds consist of positive and negative ions that are held together by strong electrostatic forces known as ionic bonds. They form when a metal reacts with a non-metal, transferring electrons, and resulting in oppositely charged particles that attract each other.
• Covalent compounds, on the other hand, are formed when non-metal atoms share pairs of electrons, creating a bond known as a covalent bond. They tend to be less soluble in water than ionic compounds, and when they do dissolve, they generally do not dissociate into ions.

Understanding the difference between these compounds is key when predicting solubility and behavior in water. Ionic compounds like KI are typically soluble in water and dissociate into their component ions. Covalent compounds like \textsf{CH}\(_3\)OH (methanol) or \textsf{I}\(_2\) (iodine), however, do not usually form ions when they dissolve in water due to the nature of covalent bonds.

Dissociation of Ionic Compounds

The concept of dissociation of ionic compounds is vital in understanding aqueous solutions and how they conduct electricity. Dissociation occurs when an ionic compound, comprised of a positive ion (cation) and a negative ion (anion), dissolves in a solvent such as water. The ionic compound breaks apart into its constituent ions, which are then surrounded by solvent molecules.

In the example of KI (potassium iodide), when it is added to water, it dissolves because the water molecules, being polar, are attracted to the positive and negative ions. The positive hydrogen ends of the water molecules surround the negative iodide ions, and the negative oxygen ends surround the positive potassium ions, pulling them away from the solid structure into the solution.

• The equation for the dissociation of KI in water is: \textsf{KI (s)} \(\rightarrow\) \textsf{K}\(^+\) (aq) + \textsf{I}\(^-\) (aq).

Such solvated ions are what make the solution capable of conducting electricity and are involved in reactions in the solution. Covalent compounds like \textsf{CH}\(_3\)OH (methanol), when dissolved in water, do not typically undergo dissociation because their atoms are bound by shared electrons, not ionic bonds.