http://www.ck12.org Chapter 16. Solutions
and gasoline. Both of these substances are nonpolar, so they are miscible and form a homogeneous mixture when
combined.
Solid Solutes
Solutions can also be comprised of a solid solute and a liquid solvent. These interactions are governed by the
same three interactions we discussed earlier: polar-polar, nonpolar-polar, and nonpolar-nonpolar. TheTable16.1
describes these interactions.
TABLE16.1:Solution Interactions
Solid (solute) Liquid (solvent) Example Result
polar polar NaCl + H 2 O Homogeneous solution
nonpolar polar I 2 + H 2 O Heterogeneous mixture
polar nonpolar NaCl + Toluene Heterogeneous mixture
nonpolar nonpolar I 2 + Toluene Homogeneous mixtureExample 16.4
Using the data in theTable16.1, could you replace the solute or the solvent in the heterogeneous mixtures with
another material to make them homogeneous?
Answer:
There are several combinations that could be described. For instance, in the polar-nonpolar mixture between NaCl
and toluene, the NaCl could be replaced with a nonpolar solid, like I 2 , or the toluene could be replaced with a polar
substance, like water.
Ionic Solids in Water
When placed in water, ionic solids dissolve to varying degrees. Some ionic solids have a high solubility in water
(e.g., NaCl), while others barely dissolve at all (e.g., AgCl). Still others are moderately soluble (e.g., Ag 2 CO 3 ).
The solubility rules we studied in the chapter onChemical Reactionsprovide guidelines for predicting the relative
solubility of a given ionic compound in water. In this chapter, we will focus primarily on water-soluble ionic solids.
When a soluble ionic solid is added to water, it interacts with water molecules and dissociates into isolated ions
that diffuse out into the solution. These charged particles become solvated by surrounding water molecules (
Figure16.6). Although the strong ionic bonds in the solid are broken up, they are replaced by numerous favorable
interactions between the charged ions and the partial charges on the appropriate ends of the polar water molecules.
Notice that for each unit of NaCl that dissolves, two particles are freed into solution, the Na+cation and the Cl−
anion. This means that if one mole of NaCl is dissolved, 2 moles of solute particles are found in the homogeneous
solution (one mole of each ion). This dissociation is quantified by something called thevan’t Hoff factor. The
van’t Hoff factor (i) describes the number of moles of solute particles that are found in a solution when one mole of
a substance is completely dissolved. The van’t Hoff factor for NaCl would be expressed as i = 2.
Example 16.5
If one mole of magnesium fluoride (MgF 2 ) is added to water and fully dissociates, how many moles of particles will
be formed?
Answer:
We can describe the dissociation of magnesium fluoride as follows: