9.1 Solutions

A solutionAnother name for a homogeneous mixture. is another name for a homogeneous mixture. Chapter 1 "Chemistry, Matter, and Measurement" defined a mixture as a material composed of two or more substances. In a solution, the combination is so intimate that the different substances cannot be differentiated by sight, even with a microscope. Compare, for example, a mixture of salt and pepper and another mixture consisting of salt and water. In the first mixture, we can readily see individual grains of salt and the flecks of pepper. A mixture of salt and pepper is not a solution. However, in the second mixture, no matter how carefully we look, we cannot see two different substances. Salt dissolved in water is a solution.

The major component of a solution, called the solventThe major component of a solution., is typically the same phase as the solution itself. Each minor component of a solution (and there may be more than one) is called the soluteThe minor component of a solution.. In most of the solutions we will describe in this textbook, there will be no ambiguity about whether a component is the solvent or the solute.) For example, in a solution of salt in water, the solute is salt, and solvent is water.

Solutions come in all phases, and the solvent and the solute do not have to be in the same phase to form a solution (such as salt and water). For example, air is a gaseous solution of about 80% nitrogen and about 20% oxygen, with some other gases present in much smaller amounts. An alloyA solid solution of a metal with other substances dissolved in it. is a solid solution consisting of a metal (like iron) with some other metals or nonmetals dissolved in it. Steel, an alloy of iron and carbon and small amounts of other metals, is an example of a solid solution. Table 9.1 "Types of Solutions" lists some common types of solutions, with examples of each.

What causes a solution to form? The simple answer is that the solvent and the solute must have similar intermolecular interactions. When this is the case, the individual particles of solvent and solute can easily mix so intimately that each particle of solute is surrounded by particles of solute, forming a solution. However, if two substances have very different intermolecular interactions, large amounts of energy are required to force their individual particles to mix intimately, so a solution does not form.

This process leads to a simple rule of thumb: like dissolves like. Solvents that are very polar will dissolve solutes that are very polar or even ionic. Solvents that are nonpolar will dissolve nonpolar solutes. Thus water, being polar, is a good solvent for ionic compounds and polar solutes like ethanol (C₂H₅OH). However, water does not dissolve nonpolar solutes, such as many oils and greases (Figure 9.1 "Solubility").

Figure 9.1 Solubility

Because of different intermolecular interactions, oil (on top) and water (bottom, colored red) do not dissolve in each other.

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We use the word solubleA solute that dissolves in a particular solvent. to describe a solute that dissolves in a particular solvent, and the word insolubleA solute that does not dissolve in a particular solvent. for a solute that does not dissolve in a solvent. Thus, we say that sodium chloride is soluble in water but insoluble in hexane (C₆H₁₄). If the solute and the solvent are both liquids and soluble in any proportion, we use the word miscibleLiquids that dissolve in each other., and the word immiscibleLiquids that do not dissolve in each other. if they are not.