The Foundations of Chemistry

EFFECT OF PRESSURE ON SOLUBILITY

Changing the pressure has no appreciable effect on the solubilities of either solids or

liquids in liquids. The solubilities of gases in all solvents increase, however, as the partial

pressures of the gases increase (Figure 14-7). Carbonated water is a saturated solution of

carbon dioxide in water under pressure. When a can or bottle of a carbonated beverage

is opened, the pressure on the surface of the beverage is reduced to atmospheric pressure,

and much of the CO 2 bubbles out of solution. If the container is left open, the beverage

becomes “flat” because the released CO 2 escapes.

Henry’s Lawapplies to gases that do not react with the solvent in which they dissolve

(or, in some cases, gases that react incompletely). It is usually stated as follows.

The pressure of a gas above the surface of a solution is proportional to the concen-

tration of the gas in the solution. Henry’s Law can be represented symbolically as

PgaskCgas

Pgasis the pressure of the gas above the solution, and kis a constant for a particular gas

and solvent at a particular temperature. Cgasrepresents the concentration of dissolved gas;

it is usually expressed either as molarity (Section 3-6) or as mole fraction (Section 14-8).

The relationship is valid at low concentrations and low pressures.

MOLALITY AND MOLE FRACTION

We saw in Section 3-6 that concentrations of solutions are often expressed as percent by

mass of solute or as molarity. Discussion of many physical properties of solutions is often

made easier by expressing concentrations either in molality units or as mole fractions

(Sections 14-9 to 14-14).

14-8

14-7

Figure 14-7 An illustration of
Henry’s Law. The solubility of a gas
(that does not react completely with
the solvent) increases with increasing
pressure of the gas above the
solution.

554 CHAPTER 14: Solutions

(a) (b)

Increase

pressure

Carbonated beverages can be used to
illustrate Henry’s Law. When the
bottle is opened, the equilibrium is
disturbed and bubbles of CO 2 form
within the liquid and rise to the
surface. After some time, an
equilibrium between dissolved CO 2
and atmospheric CO 2 is reached.