Now let's go back to a question asked at the very beginning of this lesson, and that is - how are solutions different than pure liquids? One of the ways in which they are different, is that when you add a solute to a liquid both the freezing point and boiling point of the solution change.
Water, of course, is the liquid we will be dealing with. The freezing point of pure water is 0°C. The normal boiling point of water is 100°C. But if you make a solution using water as the solvent, the freezing point of that solution will not be 0°C nor will the boiling point be 100°C.
In addition the vapor pressure of the liquid changes. Also, something called the osmotic pressure of the liquid changes and that is related to the process of osmosis. In the pages of this section we will look at each of these in turn.
The freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure are all related to one another, because the magnitude of the change depends on the concentration of solute particles. It is also dependent on the nature of the solvent. These properties are not so much dependent on the nature of, or the chemical properties of the solute that is dissolved, but simply on the number of solute particles present, whether they are ions or molecules doesn't make too much difference except in the number. It is the concentration that make the difference, not the nature. Of course, that concentration does have to take into account whether that solute dissociates and if so, how much. Because of this they are all grouped together as a set of properties, and they are called the colligative properties.
E-mail instructor: Eden Francis
Clackamas Community College