This lecture covers the modelling of water quality, focusing on carbonate chemistry reactions in aqueous solutions. It discusses the use of PHREEQC software to simulate the addition of CaO and NaOH for water softening, showing the optimal amounts to add and the resulting changes in hardness. The lecture also explores the speciation of carbonate species and the precipitation of minerals like calcite and dolomite. Practical examples and interactive simulations are used to illustrate the concepts.