This textbook introduces the elementary basics of hydrochemistry with special focus on reaction equilibria in aquatic systems and their mathematical description. 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. 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.