14.1 Solutions: An introduction
Learning Objectives
- Describe the basic properties of solutions and how they form
- Predict whether a given mixture will yield a solution based on molecular properties of its components
- Explain why some solutions either produce or absorb heat when they form
An earlier chapter of this text introduced solutions, defined as homogeneous mixtures of two or more substances. Often, one component of a solution is present at a significantly greater concentration, in which case it is called the solvent. The other components of the solution present in relatively lesser concentrations are called solutes. Sugar is a covalent solid composed of sucrose molecules, C12H22O11. When this compound dissolves in water, its molecules become uniformly distributed among the molecules of water:
The subscript “aq” in the equation signifies that the sucrose molecules are solutes and are therefore individually dispersed throughout the aqueous solution (water is the solvent). Although sucrose molecules are heavier than water molecules, they remain dispersed throughout the solution; gravity does not cause them to “settle out” over time.
Potassium dichromate, K2Cr2O7, is an ionic compound composed of colourless potassium ions, K+, and orange dichromate ions, [latex]\text{Cr}_2\text{O}_7^{\;\;2-}[/latex]. When a small amount of solid potassium dichromate is added to water, the compound dissolves and dissociates to yield potassium ions and dichromate ions uniformly distributed throughout the mixture (Figure 14.1a), as indicated in this equation:
As for the mixture of sugar and water, this mixture is also an aqueous solution. Its solutes, potassium and dichromate ions, remain individually dispersed among the solvent (water) molecules.
Exercise 14.1a
Practice using the following PhET simulation: Sugar and Salt Solutions
Water is used so often as a solvent that the word solution has come to imply an aqueous solution to many people. However, almost any gas, liquid, or solid can act as a solvent. Many alloys are solid solutions of one metal dissolved in another; for example, US five-cent coins contain nickel dissolved in copper. Air is a gaseous solution, a homogeneous mixture of nitrogen, oxygen, and several other gases. Oxygen (a gas), alcohol (a liquid), and sugar (a solid) all dissolve in water (a liquid) to form liquid solutions. Table 14.1a gives examples of several different solutions and the phases of the solutes and solvents.
| Solution | Solute | Solvent |
|---|---|---|
| air | O2(g) | N2(g) |
| soft drinks[1] | CO2(g) | H2O(l) |
| hydrogen in palladium | H2(g) | Pd(s) |
| rubbing alcohol | H2O(l) | C3H8O(l) (2-propanol) |
| saltwater | NaCl(s) | H2O(l) |
| brass | Zn(s) | Cu(s) |
Solutions exhibit these defining traits:
- They are homogeneous; that is, after a solution is mixed, it has the same composition at all points throughout (its composition is uniform).
- The physical state of a solution—solid, liquid, or gas—is typically the same a