9.3: Solubility

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Learning Objectives

Describe solutions as saturated or unsaturated by understanding solubility.

To define a solution precisely, we need to state its concentration: how much solute is dissolved in a certain amount of solvent. Words such as dilute or concentrated are used to describe solutions that have a little or a lot of dissolved solute, respectively, but these are relative terms whose meanings depend on various factors.

Solubility

There is usually a limit to how much solute will dissolve in a given amount of solvent. This limit is called the solubility of the solute. Some solutes have a very small solubility, while other solutes are soluble in all proportions. Table \(\PageIndex{1}\) lists the solubilities of various solutes in water. Solubilities vary with temperature, so Table \(\PageIndex{1}\) includes the temperature at which the solubility was determined.

Table \(\PageIndex{1}\): Solubilities of Various Solutes in Water at 25°C (Except as Noted)
Substance	Solubility (g in 100 mL of H₂O)
AgCl(s)	0.019
C₆H₆(ℓ) (benzene)	0.178
CH₄(g)	0.0023
CO₂(g)	0.150
CaCO₃(s)	0.058
CaF₂(s)	0.0016
Ca(NO₃)₂(s)	143.9
C₆H₁₂O₆ (glucose)	120.3 (at 30°C)
KBr(s)	67.8
MgCO₃(s)	2.20
NaCl(s)	36.0
NaHCO₃(s)	8.41
C₁₂H₂₂O₁₁ (sucrose)	204.0 (at 20°C)

If a solution contains so much solute that its solubility limit is reached, the solution is said to be saturated, and its concentration is known from information contained in Table \(\PageIndex{1}\). If a solution contains less solute than the solubility limit, it is unsaturated. Under special circumstances, more solute can be dissolved even after the normal solubility limit is reached; such solutions are called supersaturated and are not stable. If the solute is solid, excess solute can easily recrystallize. If the solute is a gas, it can bubble out of solution uncontrollably, like what happens when you shake a soda can and then immediately open it.

Most solutions we encounter are unsaturated, so knowing the solubility of the solute does not accurately express the amount of solute in these solutions. There are several common ways of specifying the concentration of a solution that will be discussed in later sections.

Solution Equilibrium

When a solute dissolves, its individual atoms, molecules, or ions interact with the solvent, become solvated, and are able to diffuse independently throughout the solution (Figure \(\PageIndex{1a}\)). This is not, however, a unidirectional process. If the molecule or ion happens to collide with the surface of a particle of the undissolved solute, it may adhere to the particle in a process called crystallization. The equilibrium between dissolution and crystallization continue as long as excess solid is present, resulting in a dynamic equilibrium analogous to the equilibrium that maintains the vapor pressure of a liquid. We can represent these opposing processes as follows:

\[ solute + solvent \underset{crystallization}{\stackrel{dissolution}{\longrightleftharpoons}} solution \nonumber \]