<p>This textbook introduces the elementary basics of hydrochemistry with special focus on reaction equilibria in aquatic systems and their mathematical description. </p> <p>Topics discussed in this textbook include: structure and properties of water, concentration measures and activities, colligative properties, basics of chemical equilibria, gas-water partitioning, acid/base reactions, precipitation/dissolution, calco-carbonic equilibrium, redox reactions, complex formation, and sorption. </p> <p>Examples within the text as well as problems to be solved by the reader support the acquisition of knowledge. Complete and detailed solutions to the problems are given in a separate chapter. </p>