This section will focus on what you need to know about solutions, solution concentrations, and colligative properties in order to be successful on the SAT II Chemistry test. This material is closely tied in with the material from the first half of this chapter and “The Structure of Matter.”
Properties of Solutions
A solution is a homogenous mixture of two or more substances that exist in a single phase. There are two main parts to any solution. The solute is the component of a solution that is dissolved in the solvent; it is usually present in a smaller amount than the solvent. The solvent is the component into which the solute is dissolved, and it is usually present in greater concentration. For example, in a solution of salt water, salt is the solute and water is the solvent. In solutions where water is the solvent, the solution is referred to as an aqueous solution.
A solution does not have to involve liquids. For instance, air is a solution that consists of nitrogen, oxygen, carbon dioxide, and other trace gases, and solder is a solution of lead and tin. The general rule of thumb for solutions is the idea that like dissolves like. Polar, ionic substances are soluble in polar solvents, while nonpolar solutes are soluble in nonpolar solvents. For example, alcohol and water, which are both polar, can form a solution and iodine and carbon tetrachloride, which are both nonpolar, make a solution. However, iodine will not readily dissolve in polar water.
In a solution, the particles are really small—anywhere from 0 to 100 nm. They never settle on standing, they cannot be separated by filtering, and light will pass through a solution unchanged. One type of mixture that is not a solution is known as the colloid. In a colloid, particles are between 100 and 1000 nm in size—still too small for our eyes to distinguish, but particles this small will not settle. As is the case in solutions, the particles cannot be filtered, but they do scatter light. Some examples of colloids include gelatin, fog, smoke, and shaving cream. Another type of mixture that is not considered a solution is known as a suspension. Suspensions have much larger particles: usually over 1000 nm. Particles in a suspension will settle on standing, can often be separated by a filter, and may scatter light, but they are usually not transparent. Some examples of suspensions are muddy water, paint, and some medicines, like Pepto-Bismol.
The Solution Process
In order for a solute to be dissolved in a solvent, the attractive forces between the solute and solvent particles must be great enough to overcome the attractive forces within the pure solvent and pure solute. The solute and the solvent molecules in a solution are expanded compared to their position within the pure substances.
The process of expansion, for both the solute and solvent, involves a change in the energy of the system: this process can be either exothermic or endothermic. After dissolving, the solute is said to be fully solvated (usually by dipole-dipole or ion-dipole forces), and when the solvent is water, the solute is said to be
