7.18: Concentrations: Qualitative Comparisons

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Learning Objectives

Define concentrated.
Define dilute.

The previous six sections of this chapter have presented and applied the equations that are used to calculate the mass percent, the volume percent, the mass/volume percent, and the molarity of a solution. Each of these concentrations quantitatively ratios the amount of solute that is contained in a solution to the amount of solution that is present, overall. This generic definition of concentration is represented in the equation that is shown below.

\(\text{Concentration (C)}\) = \( \dfrac{ \rm{Amount \; of \; Solute}}{\rm{Amount \; of \; Solution}}\)

When the concentrations of multiple solutions are compared to one another, each homogeneous mixture can be qualitatively described as being "concentrated" or "dilute," relative to the other solution. Because the meanings of these terms are defined based on the relative comparisons of two or more solutions, neither of these words is quantitative on its own. If a large amount of solute is dissolved in a particular amount of solvent, the resultant solution can likely be described as concentrated. In order to be classified as dilute, a solution must contain less solute in the same amount of solvent or the same amount of solute in a larger amount of solvent, relative to the solution to which it is being compared.

The impact of these changes on the concentration of a solution can be evaluated using the generic equation that is shown above. If the quantity of solvent and, therefore, the denominator in this equation, is unchanged, reducing the amount of solute that is present lessens the value of the numerator in this proportion, and, consequently, the concentration of the solution decreases. When the amount of solute and, therefore, the numerator in this equation, remains constant, increasing the quantity of solution that is present raises the value in the denominator of this ratio, which, in turn, decreases the concentration of the solution. Therefore, compared to a more concentrated solution, a solution that is classified as "dilute" must have a smaller relative concentration. For example, a 5.2 M solution is more concentrated than a 1.7 M solution, but is more dilute than a 6.4 M solution.

Note that a concentrated solution should not be described as "strong" or assumed to be saturated, and a dilute solution should not be equated to being "weak." As discussed previously, the words "strong" and "weak" refer to the extent to which a solute dissociates, or separates, into ions during the solvation process. Since the concentration of a solution is solely dependent on the amounts of chemicals that are present, the type of solute, which dictates the extent to which that chemical dissociates, does not impact whether a solution is concentrated or dilute. Finally, a saturated solution must, by definition, contain exactly the maximum amount of solute that can be dissolved in a specified amount of solvent. Therefore, while a saturated solution is most likely concentrated, a concentrated solution is not necessarily saturated.