Chapter 7
Section 3
Objectives
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Explain the meaning of solubility and compare the solubilities of various
substances.
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Describe dilute, concentrated, saturated, and supersaturated solutions.
•
Relate changes in temperature and pressure to changes in solubility of solid and
gaseous solutes.
Express the concentration of a solution as molarity, and calculate the molarity of a
solution given the amount of solute and the volume of the solution
Solubility in Water
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Solubility is the maximum amount of a solute that will dissolve in a given
quantity of solvent at a given temperature and pressure.
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Some substances, such as oil, are insoluble in water, meaning they never
dissolve.
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Other substances are said to be soluble in water because they dissolve
easily in water.
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However, there is often a limit to how much of a substance will
dissolve.
Different substances have different solubilities.
•
To express how much of a substance can dissolve in a solvent, you need to use
the concentration.
•
Concentration is the amount of a particular substance in a given volume of
solution.
•
•
A solution whose ratio of solute to solvent is relatively high is referred to
as concentrated.
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A solution whose ratio of solute to solvent is relatively low is referred to
dilute.
An unsaturated solution contains less than the maximum amount of solute that
can dissolve.
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A saturated solution is at a point where no more solute can be dissolved under
the same conditions.
•
•
If you add more solute to a saturated solution, it will simply fall to the
bottom of the container.
A supersaturated solution holds more dissolved solute than is required to
reach equilibrium at a given temperature.
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To make a supersaturated solution, you raise the temperature of a
solution, dissolve more solute, then let the solution cool again.
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Gases can also dissolve in water.
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Unlike solid solutes, gaseous solutes are less soluble in warmer water than they
are in colder water.
•
•
Example: Soda goes flat quickly at room temperature.
The solubility of gases also depends on pressure. Lowered pressure of gas
above a solution leads to dissolved gas bubbling out of the solution.
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Example: When a can of soda is opened, carbon dioxide gas that had
been dissolved in the soda bubbles out of solution.
•
Example: If a scuba diver surfaces too quickly, dissolved nitrogen gas in
the bloodstream bubbles out of solution, which causes a painful condition
called the bends.
Concentration of Solutions
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There are several ways to express the concentration of solutions, but one of the
most useful ways is by using molarity: moles of dissolved solvent per liter
of solution.
•
Note that molarity is moles per liter of solution,
not per liter of solvent.
•
A 1.0 M, which is read as “one molar,” solution of NaCl, contains 1.0 mol of
dissolved NaCl in every
1.0 L of solution.
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Other measures of solution concentration can be used.
•
These include:
•
mass percent (grams of solute per 100 g
of solution)
•
•
Ingredients in many food and household products use mass
percent.
parts per million (grams of solute per 106 g
of solution)
•
Used for very small concentrations, such as for
environmental regulations.