Why is it that some substances readily mix to form solutions while others do not?
Whether one substance dissolves in another substance is largely dependent on the inter-molecular forces present in the substances.
For a solution to form, the solute particles must become dispersed throughout the sol-vent. This process requires the solute and solvent to initially separate and then mix.
Another way of thinking of this is that the solute particles must separate from each other and disperse throughout the solvent.
The solvent may separate to make room for the solute particles or the solute particles may occupy the space between the solvent particles. Determining whether one sub-stance dissolves in another requires exam-ining three different intermolecular forces present in the substances—between the
solute particles, between the solvent parti-cles, and between the solute and solvent par-ticles. The formation of a solution can occur when the magnitude of all three of these forces are similar. Conversely, if the inter-molecular forces for solute particles are much stronger than their attraction for sol-vent particles, then mixing is not favored.
The tendency of substances with similar intermolecular forces to mix leads to the general rule of thumb: “like dissolves like.”
Substances with “like” intermolecular forces tend to form solutions. As a simple analogy for the solution process, assume two situations take place in a stadium in a town with two rival teams. In the first situ-ation, consider a football game between the two cross-town rivals. The fans from one side of town will occupy one side of the sta-dium, while the fans from the opposite side of town will sit together on the other side.
There is no attraction of either side for the other, and this corresponds to the situation when two substances do not mix to form a solution. On the other hand, consider a sec-ond situation in which a rock concert is held in the stadium. In this case, people from the Table 11.2
Examples of Solutions
opposite sides of towns will mix throughout the stadium. Fans from both sides of town are equally attracted to the music, and there is an equal desire to get the best seats. This second situation is analogous to what hap-pens when two substances mix to form a solution.
The “like dissolves like” rule provides a general guideline for determining which type of substances will form solutions. To see how this rule applies, let’s look at the solubility of some of the major types of compounds. Ionic compounds tend to dis-solve in polar dis-solvents. Because water is the most common polar solvent, ionic com-pounds tend to form aqueous solutions. For example, NaCl dissolves in water. Consider what happens when a few grains of salt are sprinkled into water. Each salt grain is a crystalline lattice structure containing mil-lions of unit cells of NaCl. Although the ionic bond holding the NaCl unit cells together is strong, the polarity of water causes the positive hydrogen end of water to be attracted to the chloride ion (Cl–) and the negative oxygen end to be attracted to the
sodium ion (Na). Water molecules initially interact with the NaCl crystals on the outer surface of the grains. Water molecules act collectively to pull individual ions out of the crystalline structure (Figure 11.1). The gen-eral process by which a solute becomes dis-persed in a solution is known as solvation.
When water is the solvent, the process is known as hydration.
While many ionic compounds are solu-ble in water, many are not. The term “solu-bility” is somewhat subjective. There are actually degrees of solubility. A substance is considered soluble if 0.1 moles of it can dis-solve in 1 liter of water. If less than 0.001 mole of the substance dissolves in water, a substance is considered insoluble. Partially soluble substances fall between these two extremes. Table 11.3 summarizes the solu-bility of some major groups of ionic com-pounds in water.
Whether an ionic compound dissolves in water depends on the strength of the ionic bond holding the compound together.
Water must have sufficient strength to break the ionic bond. The strength of the
Figure 11.1
Diagram of Hydration of NaCl (Rae Déjur)
bond depends on the size and charge of the ions in the compound. Stronger ionic bonds occur when the ions are smaller and the ions carry multiple charges. Because of this, many ionic compounds may not be sol-uble in water, as is seen by noting the exceptions listed in Table 11.3.
Polar covalent molecules may or may not dissolve in water depending on whether they have the ability to form hydrogen bonds. For example, many alcohols will dis-solve in water because the OH group char-acteristic of alcohols gives them the ability to hydrogen bond with water. The attraction between alcohol and water is demonstrated when equal volumes of the two are mixed to give a total less than the sum of their indi-vidual volumes, for example, 100 mL of alcohol 100 mL of water produces less than 200 mL of solution. Nonpolar sub-stances tend not to dissolve in water, but they do dissolve in nonpolar solvents. Fats, oil, grease, and gasoline, for example, do not dissolve in water, but they form a layer on top of water. When substances do not mix but form distinct layers, they are referred to as immiscible. Nonpolar
sub-stances can be dissolved in a nonpolar sol-vent such as benzene or carbon tetrachlo-ride.
Electrolytes
A useful characteristic in classifying solutions is their ability to conduct electric-ity. Solutions that are good conductors of electricity are said to contain strong elec-trolytes. Strong electrolytes have the ability to produce ions when dissolved in water; the greater the degree of ionization of a sub-stance is, the stronger the electrolyte. Strong electrolytes include soluble salts, strong acids, and strong bases. Many substances, such as weak acids and weak bases, only par-tially ionize when dissolved in water. These substances are referred to as weak elec-trolytes. Still other substances do not ionize at all, but they dissolve as whole molecules and are referred to as nonelectrolytes. Sugar is an example of a nonelectrolyte. When sugar dissolves, the covalent bonds holding the molecule together are sufficiently strong to keep the molecule together. Table sugar, C12H22O11, contains a number of oxygen Table 11.3
Solubility of Ionic Compounds in Water at 25°C
atoms making it a polar molecule. Its high number of oxygen atoms allows it to hydro-gen bond to many water molecules, and therefore, sugar is quite soluble.
Soluble ionic compounds tend to be strong electrolytes, while alcohols and organic compounds are nonelectrolytes.
Remember that classification as a strong electrolyte, weak electrolyte, or nonelec-trolyte is somewhat subjective. Freshwater can be either a weak electrolyte or a non-electrolyte depending on its purity. The important consideration in classifying a sub-stance is to what extent an aqueous solution of the substance will conduct electricity.