This unit on the principles of equilibrium is one of 10 first year units produced by the Independent Learning Project for Advanced Chemistry (ILPAC). The unit consists of two levels. After a treatment of non-mathematical aspects in level one (the idea of a reversible reaction, characteristics of an equilibrium state, the Le Chatelier's principle), the equilibrium law is introduced in level two. Students are shown step-by-step how to solve numerical problems based on the law as well as applications of the law to such systems as the equilibrium between a slightly soluble salt and its saturated solution. Five experiments, exercises (with answers), and pre- and post-tests are included. More advanced topics (such as the relationship between equilibrium constant and changes in enthalpy and free energy) are also included to meet the needs of certain syllabi. Students completing this ILPAC unit assume a greater responsibility for their own learning and can work, to some extent, at their own pace. By providing guidance, and detailed solutions to exercises in the units, supported by the optional use of video-cassettes, the project allows students to study A-level chemistry with less teacher contact time than a conventional course demands. (JN)