Chemistry, Third edition

11 · SOLUTIONS AND SOLUBILITY

Dynamic nature of dissolution

Equilibrium involved in dissolving substances

Think of a beaker containing saturated potassium iodide solution, with undissolved

(solid) K,Iat the bottom of the beaker. Experiments show that although the con-

centration of dissolved KI remains fixed at that temperature, the ions in the solution

and in the solid are continuously exchanging places. Suppose at an instant, a pair of

K(aq) and I(aq) ions come together and precipitate at the bottom of the beaker.

Simultaneously, a pair of Kand Iions in the solid break up into the solution. In

other words, a dynamic equilibriumexists between the iodide in the solid and in solu-

tion. We represent this as:

K,I(s) K(aq) + I(aq)

Examples of dynamic equilibria in solution are not restricted to the dissolving of

ionic compounds. As we have seen, a mixture of hexane and water consists of two

layers, one containing a small amount of water in hexane and the other a small

amount of hexane in water. In fact, the dissolved hexane and water molecules are

continuously exchanging places with the bulk solvents and we can write:

water in water layer water in hexane layer

also

hexane in hexane layer hexane in water layer

Other examples of dynamic equilibria we shall discuss in this chapter include the

dissolving of gases in water and the distribution of a solid between two solvents.

Solubility of sparingly soluble ionic

compounds

Molar solubility of ionic compounds

It is convenient to define the solubility of ionic substances in units of g per 1000 g of

saturatedsolution, or as the number of moles of substance needed to produce 1 dm^3

of saturated solution.Molar solubilityis defined as follows:

The molar solubility of a compound is its concentration (mol dm^3 ) in a

saturated solution at that temperature.

Molar solubility is given the symbol s.

Ionic compounds which are only slightly solublein water are said to be sparingly

soluble. Examples include silver chloride (Ag,Cl) and barium sulfate

(Ba^2 ,SO 42 ). Since such compounds are only slightly soluble, the volume of water

used to dissolve a salt may be taken to be equal to the volume of the solution. (For

example, if we dissolve 0.0010 g of silver chloride in 1000 cm^3 of water, we may safely

assume that the volume of the solution is also 1000 cm^3 .) This approximation does

11.3

11.2

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