COLLIGATIVE PROPERTIES OF SOLUTIONS
There are four colligative properties of solutions:
- • Boiling-point elevation
- • Freezing-point depression
- • Vapor-pressure lowering
- • Osmotic pressure
The term “colligative” means having to do with the collection or number of particles and/or ions dissolved in solution. It is an oversimplification, since size and polarity also play a role, albeit a less important one, in the chemistry of solutions. Colligative properties depend on the concentration of the solute (in units of molality) and its “i” factor (van’t Hoff factor), that is, the total number of dissolved particles. The “i” factor depends on the specific chemical formula of the solute as well as the type of solute, as discussed below under “electrolytes.”
Before describing each of the four colligative properties or effects in detail, it is useful to review the concept of a solution. Recall that a solution is a physical state wherein a solute has been dissolved completely and uniformly in a solvent, forming a solution of uniform composition. Unless specified otherwise, the solvent is assumed to be water, whose normal boiling point, freezing point, and density are well known. In the equations for each of the four properties, the concentration of the solution appears, although concentration units may vary. It is critically important, when doing calculations with solutions, to distinguish among units of mass, volume, and density for solute, solvent, and solution, respectively.
For solutions, there are three types of solutes or “electrolytes”:
- • Strong electrolytes
- • Nonelectrolytes
- • Weak electrolytes
Strong electrolytes are generally ionic compounds, such as salts, and strong acids and bases, that dissociate and ionize completely (i.e., 100%) in water. The solutions they form with water become strong conductors of electric currents. Examples include NaCl, KNO3, CaCb, HCl, and NaOH. Thus, 1 mole of NaCl produces 2 moles of ions or particles; 1 mole of CaCl2 produces 3 moles of ions or particles (1 mole of Ca2+ ions and 2 moles of Cl- ions). The van’t Hoff factor is the total number of ions or particles per formula unit or mole: 2.0 for NaCl, 3.0 for CaCh, and so on. Experimentally measured van’t Hoff factors are generally less than theoretically predicted ones for a given solute.
Nonelectrolytes are generally alcohols and sugars, such as ethanol (C2H5OH) and glucose (C6H12O6), or other organic substances that dissolve in water but do not ionize. One mole of a nonelectrolyte always produces 1 mole of particles. Thus, the van’t Hoff factor is 1.0.
A weak electrolyte, such as acetic acid, is a substance that is only partially dissociated and ionized, typically 10% or less, leaving 90% or more undissociated and thereby behaving much like a nonelectrolyte. Its van’t Hoff factor is greater than 1 but less than the total number of ions available for dissociation.