Nearly every chemical reaction takes place in homogeneous mixtures called solutions. Therefore, we must understand the properties of solutions before we can even begin to understand those reactions. Perhaps the most salient characteristic of a solution is its concentration--how much solute is dissolved in what amount of solvent. Several different units of concentration like mass percent, mole fraction, molarity, normality, and molality have been developed for use in different situations. Solution Composition explains the definitions and uses of those units and why it is necessary to have so many different units of concentration.

After we have discussed the units of concentration, we will explore the questions of why solutions form at all and what factors affect the solubility of solutes in different solvents. As we shall see, like dissolves like. Non-polar solvents dissolve non-polar solutes better than polar solvents and polar solvents dissolve polar solutes better than non-polar solvents. Raising the temperature of a solution will increase the solubility of most solid solutes. Likewise, according to Henry's law, increasing the pressure above a solution will increase the solubility of most gaseous solutes.

There are several other properties of solutions, besides concentration and solubility, called colligative properties, that depend only on the number of solute particles, not on their identities. The colligative properties that we will study are vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. Each of those properties is discussed in detail in Colligative Properties.