7.12: Concentrations: Introduction
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The previous three sections of this chapter have defined and applied concepts related to solubility, which is defined as the maximum amount of a solute that can dissolve in a given amount of solvent. Determining these maximum values is important, so that chemists do not waste materials by adding excess solute when preparing solutions. However, saturated solutions are not often utilized when performing chemical reactions, as measuring exactly the maximum amount of solute that should be dissolved in a specified amount of solvent can be tedious and time-consuming. Additionally, due to the large quantity of solute that is normally incorporated into a saturated solution, these homogeneous mixtures are often both expensive to produce and dangerous to handle.
As a result, chemists often use unsaturated solutions, which contain less than the maximum amount of solute that can dissolve in a specified amount of solvent, during experimental trials. However, the quantity of solute that is contained in an unsaturated solution cannot be described using a solubility limit, which must, by definition, correspond to a maximum amount of solute. Therefore, in order to indicate the amount of solute that is dissolved in a given amount of solvent, chemists have defined several different types of concentrations (C), which each use a different chemically-acceptable unit, or combination of units, in their corresponding calculations.
While solubilities and concentrations are inherently-related concepts, these terms should not be interchanged, as their meanings, applications, and representational formats are all subtly different. A solubility limit corresponds to the maximum amount of a solute that can dissolve in a given amount of solvent. In contrast, a concentration indicates the actual amount of solute, which may or may not correspond to a maximum quantity, that is dissolved in a given amount of solvent. Therefore, while solubilities are primarily applied in a theoretical context, concentrations are used more often in practical experimentation. Finally, both solubilities and concentrations are initially expressed as proportions. However, solubility limits are subsequently applied as conversion factors, and the resultant calculated quantities should not be rounded, because solubility limits correspond to the exact amount of solute that can dissolve in a corresponding amount of solvent. In contrast, concentrations are rarely used as conversion factors and, instead, are usually simplified and presented as "end-result" values. Because concentrations are not defined as exact quantities, their corresponding calculated values should be expressed using the correct number of significant figures.