In chemical engineering, theories serve as the foundational framework for understanding and manipulating processes at the molecular level to achieve desired outcomes. These theories encompass principles from physics, chemistry, and mathematics, applied to phenomena like mass and energy transfer, reaction kinetics, and thermodynamics. For instance, the mass balance principle ensures conservation of mass in chemical processes, while thermodynamic theories elucidate energy transformations and equilibria. Kinetic theories provide insight into reaction rates and mechanisms. Understanding these theories is paramount for designing efficient processes, optimizing reaction conditions, and developing novel materials. How do these theories interplay in your work, and what challenges do you encounter in applying them?