- Strong electrolytes consist of solutes that dissociate completely in solution. Strong acids, strong bases, and soluble salts are in this category. (We will discuss acids and bases in chapter 6.)
- Nonelectrolytes are substances that are predominantly covalently bonded, generally will not produce ions in solution, and therefore are considered nonconductors.
- Weak electrolytes consist of solutes that dissociate only a little in solution. Weak acids, weak bases, and slightly soluble salts are in this category.
The greater the degree of dissociation of the solute, the greater the conductivity of the solution. Consider two acid solutions that have the same concentration—hydrochloric acid and acetic acid. Hydrochloric acid ionizes completely, while only about 2% of the acetic acid molecules ionize. If a conductivity apparatus were used to test the two solutions, HCl would conduct an electric current to a much greater degree because there is more available charge in solution. Below is a figure showing the ionization of barium chloride; as you can see, the Ba+ and Cl- ions are floating free in solution, and this makes barium chloride an electrolyte.
Colligative Properties
Properties of solutions that depend on the number of solute particles present per solvent molecule are called colligative properties. The concentration of solute in a solution can affect various physical properties of the solvent including its freezing point, boiling point, and vapor pressure. For the SAT II you will only need to be familiar with the first two.
Freezing Point Depression
The freezing point of a substance is defined as the temperature at which the vapor pressure of the solid and the liquid states of that substance are equal. If the vapor pressure of the liquid is lowered, the freezing point decreases.
Why is a solution’s freezing point depressed below that of a pure solvent? The answer lies in the fact that molecules cluster in order to freeze. They must be attracted to one another and have a spot in which to cluster; if they act as a solvent, solute molecules get in the way and prevent them from clustering tightly together. The more ions in solution, the greater the effect on the freezing point. We can calculate the effect of these solute particles by using the following formula:
DTf = Kf 
msolute 
i
where
DTf = the change in freezing point
Kf = molal freezing point depression constant for the substance (for water = 1.86ºC/m)
m = molality of the solution
i = number of ions in solution (this is equal to 1 for covalent compounds and is equal to the number of ions in solution for ionic compounds)
Boiling Point Elevation
As you learned earlier in this chapter, the boiling point of a substance is the temperature at which the vapor pressure equals atmospheric pressure. Because vapor pressure is lowered by the addition of a nonvolatile solute, the boiling point is increased. Why? Since the solute particles get in the way of the solvent particles trying to escape the substance as they move around faster, it will take more energy for the vapor pressure to reach atmospheric pressure, and thus the boiling point increases. We can calculate the change in boiling point in a way that’s similar to how we calculate the change in freezing point:
