The colligative properties of solutions consist of freezing point depression, boiling point elevation, vapor pressure lowering, and osmotic pressure. These properties ideally depend on changes in the entropy of the solution on dissolving the solute, which is determined by the number of the solute molecules or ions but does not depend on their structure. This resource presents the derivations of the colligative properties and an analysis of the assumptions made. In particular, it concentrates on water and aqueous solutions, where deviations from ideal behavior provide useful insights into the interactions of molecules and ions with water. The materials also include graphic representations of the relationships being described. Links to additional information are embedded in the text.