7.2.1: Solutions

Learning Objectives

• To understand what causes solutions to form.

A solution is another name for a homogeneous mixture. 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 solvent, 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 solute. 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 alloy 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 \(\PageIndex{1}\) lists some common types of solutions, with examples of each.

Table \(\PageIndex{1}\): Types of Solutions

Solvent Phase	Solute Phase	Example
gas	gas	air
liquid	gas	carbonated beverages
liquid	liquid	ethanol (C2H5OH) in H2O (alcoholic beverages)
liquid	solid	saltwater
solid	gas	H2 gas absorbed by Pd metal
solid	liquid	Hg(ℓ) in dental fillings
solid	solid	steel alloys

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 solvent, 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. Thus two alkanes like n-heptane, C₇H₁₆, and n-hexane, C₆H₁₄, are completely miscible in all proportions. The C₇H₁₆ and C₆H₁₄ molecules are so similar (recall Section 4.6) that there are only negligible differences in intermolecular forces.

For a similar reason, methanol, CH₃OH, is completely miscible with water. In this case both molecules are polar and can form hydrogen bonds among themselves, and so there are strong intermolecular attractions within each liquid. However, CH₃OH dipoles can align with H₂O dipoles, and CH₃OH molecules can hydrogen bond to H₂O molecules, and so the attractions among unlike molecules in the solution are similar to those among like molecules in each pure liquid.

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 \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): A beaker holds water with blue food dye (upper liquid layer) and a much more dense perfluoroheptane (a fluorocarbon) lower liquid layer. The two fluids cannot mix and the dye cannot dissolve in fluorocarbon. A goldfish and a crab have been introduced into the water. The goldfish cannot penetrate the dense fluorocarbon. The crab floats at the liquid boundary with only parts of his legs penetrating the fluorocarbon fluid, unable to sink to the bottom of the beaker. Quarter coins rest on the bottom of the beaker. Animals were rescued from their predicament after the photo was taken. Figure used with permission from Wikipedia (Sbharris (Steven B. Harris)).

We use the word soluble to describe a solute that dissolves in a particular solvent, and the word insoluble 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 miscible, and the word immiscible if they are not.

Example \(\PageIndex{1}\)

Water is considered a polar solvent. Which substances should dissolve in water?

1. methanol (CH₃OH)
2. sodium sulfate (Na₂SO₄)
3. octane (C₈H₁₈)

Solution

Because water is polar, substances that are polar or ionic will dissolve in it.

1. Because of the OH group in methanol, we expect its molecules to be polar. Thus, we expect it to be soluble in water. As both water and methanol are liquids, the word miscible can be used in place of soluble.
2. Sodium sulfate is an ionic compound, so we expect it to be soluble in water.
3. Like other hydrocarbons, octane is nonpolar, so we expect that it would not be soluble in water.

Exercise \(\PageIndex{1}\)

Toluene (C₆H₅CH₃) is widely used in industry as a nonpolar solvent. Which substances should dissolve in toluene?

1. water (H₂O)
2. sodium sulfate (Na₂SO₄)
3. octane (C₈H₁₈)

Answer

Octane only.

Example \(\PageIndex{2}\)

Predict which of the following compounds will be most soluble in water:

1. \(\underset{\text{Ethanol}}{\mathop{\text{CH}_{\text{3}}\text{CH}_{\text{2}}\text{OH}}}\,\)
2. \(\underset{\text{Hexanol}}{\mathop{\text{CH}_{\text{3}}\text{CH}_{\text{2}}\text{CH}_{\text{2}}\text{CH}_{\text{2}}\text{CH}_{\text{2}}\text{CH}_{\text{2}}\text{OH}}}\,\)

Solution

Since ethanol contains an OH group, it can hydrogen bond to water. Although the same is true of hexanol, the OH group is found only at one end of a fairly large molecule. The rest of the molecule can be expected to behave much as though it were a nonpolar alkane. This substance should thus be much less soluble than the first. Experimentally we find that ethanol is completely miscible with water, while only 0.6 g hexanol dissolves in 100 g water.

Exercise \(\PageIndex{2}\)

Would I₂ be more soluble in CCl₄ or H₂O?

Answer

I₂ is nonpolar. Of the two solvents, CCl₄ is nonpolar and H₂O is polar, so I₂ would be expected to be more soluble in CCl₄.

Key Takeaway

• Solutions form because a solute and a solvent experience similar intermolecular interactions.