This unit on equilibrium is one of 10 first year units produced by the Independent Learning Project for Advanced Chemistry (ILPAC). The unit, which consists of two levels, focuses on the application of equilibrium principles to equilibria involving weak acids and bases, including buffer solutions and indicators. Level one uses Le Chatelier's principle to provide a qualitative explanation of the behavior of equilibria involving the weak acids and bases. Key ideas, including Bronsted-Lowry theory, self-ionization of water, and action of buffer solutions are introduced. Level two covers much of the same material, but from a more mathematical standpoint. A discussion of the way in which pH changes during different types of acid-base titration, and the choice of indicators for such titrations is included. Eight experiments, exercises (with answers), and pre- and post-tests are provided. 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)