Soil is a complex, multi-component, and polydisperse system with multiple phases. It consists of a solid component and pores (comprising the solid phase), which are filled with soil solution (forming the liquid phase), as well as soil air (representing the gaseous phase), and living organisms. Most soils are primarily composed of minerals. The solid phase, mainly consisting of mineral substances, makes up 40-65% of the soil volume (or 90-99% or more of its mass). In soil solutions, numerous chemical reactions take place, including competitive reactions such as complex formation and protonation, among others. Under these conditions, a single element can exist in multiple ionic forms, either as a free ion or as part of various complex ions. Consequently, the concentration of any ionic form the variations in the composition and concentration of soil solutions can be substantial. However, despite the inherent heterogeneity of soil cover, these variations generally fall within the bounds characteristic of soils of a given type. This study explores the application of chemical thermodynamics and computer-based equilibrium composition calculations in describing chemical reactions in “mineral-soil solution” systems. A limitation of instrumental physicochemical methods for analyzing solutions is that they either characterize the overall composition or ion activity or do not provide insights into the equilibrium distribution of various chemical forms of elements. It is known that most metal ions in soil solutions exist in ionic forms, complexes with mineral acid anions, and organic compounds. Furthermore, the bioavailability and toxicity of chemical elements, especially metals, depend on the chemical form in which they are present in the soil solution. Therefore, to assess the equilibrium distribution of chemical forms of elements in soil solutions, a combination of analytical, thermodynamic and computational methods is necessary. Consequently, thermodynamic calculations reveal that the state of several elements, especially metal ions prone to forming complexes, varies significantly in solutions with different pH levels. Chemical equilibrium processes in the “mineral-water solution” system have been described by a generalized equation of reactions, where water-soluble species are assumed to be formed in quantities proportional to their partial molar fractions. Mathematical expressions have been derived to calculate the change in Gibbs energy for overall processes, taking into account side reactions such as hydrolysis and complex formation under real, non-standard conditions. The developed approach for calculating Gibbs energy has been successfully applied to chemical processes in soil involving kaolinite and montmorillonite. This method is versatile and can be applied to any heterogeneous processes in aqueous systems involving the formation of a solid phas