16.1. Solubility http://www.ck12.org
solubility of NaCl is minimally influenced by temperature (its solubility curve is nearly flat). For gaseous solutes,
solubilitydecreasesat higher temperatures. We will look more at this effect later in the lesson.
Pressure
Higher pressures increase the solubility of gases. You are probably familiar with this concept as it relates to
carbonated beverages. Before opening the container, the inside is pressurized, so a large amount of CO 2 is dissolved
in the liquid. After opening, the pressure decreases (to the ambient pressure), so the solubility of CO 2 drops, causing
it to bubble out of solution. Because they are not compressible like gases, solid and liquid solutes do not have
noticeable changes in solubility at different external pressures.
A Review of Intermolecular Forces
Our understanding of the behavior of solutes and solvents can be largely explained at the molecular level using our
model of intermolecular forces. Some substances will mix freely while others barely mix at all. This is due to the
interactions between particles of the solvent and solute. Recall that nonpolar molecular substances are held together
in the solid and liquid phases by relatively weak London dispersion forces, in which induced dipoles line up into a
favorable arrangement. An example of this is the interactions found between molecules of iodine (I 2 ). In contrast,
polar molecules are held together by stronger dipole-dipole interactions. Additionally, molecules that contain N-H,
O-H, or F-H bonds exhibit a special dipole-dipole interaction called hydrogen bonding, which is unusually strong
even for a polar interaction. Ammonia (NH 3 ) and water are examples of small molecules that exhibit hydrogen
bonding. The cations and anions in an ionic compound are held together by very strong ionic bonds, but ion-dipole
interactions are nearly as strong. Ion-dipole interactions would be found, for example, when an ionic substance like
NaCl is dissolved in water. Each ion is attracted to the appropriate end of the dipole on surrounding molecules of
water.
Liquid Solutes
When combining two liquids, we can generally predict whether they will mix to form a homogeneous solution or
not by looking at the relative polarity of each substance. We will consider three scenarios: the combination of two
polar liquids, the combination of one polar and one nonpolar liquid, and the mixing of two nonpolar liquids.
Polar-Polar Interactions
Polar-polar interactions occur when two or more polar liquids are mixed. An example of this is when methanol
mixes with water. Both of these are small polar molecules containing O-H bonds, which means that they can both
participate in hydrogen bonding. Figure16.3 shows molecules of methanol and water connected by hydrogen
bonds. Because of these strong interactions, the two substances mix freely to form a homogeneous mixture; they
aremiscible. One way to remember this interaction is the phrase "like dissolves like." In this case, a polar solvent
dissolves polar solutes.
Nonpolar-Polar Interactions
Toluene (C 6 H 5 CH 3 is an organic compound that is often used as a solvent in paint thinners. Toluene is a nonpolar
compound. When mixed with water, the two substances will separate into two layers rather than forming a homo-
geneous solution; these two liquids areimmiscible. Toluene is a nonpolar chain that cannot form hydrogen bonds