Liquid cell electron microscopy enables direct in situ imaging of processes in liquids and objects suspended in liquids with nanoscale resolution. However, the irradiating electrons affect the chemistry of the suspending medium, typically an aqueous solution, producing molecular and radical products such as hydrogen, oxygen, and hydrated (solvated) electrons. These may impact the imaged structures and phenomena. A good understanding of the interactions between the electrons and the irradiated medium is necessary to correctly interpret experiments, minimize artifacts, and take advantage of the irradiation. We predict the composition of water subjected to electron irradiation in the electron microscope. We reinterpret available experimental data, such as beam-induced variations in pH and colloid aggregation, in light of our predictions and show new observations of crystallization and etching as functions of dose rate, resolving conflicting reports in the scientific literature. We make our computer code available to readers. Our predictive model is useful for designing experiments that minimize unwanted beam effects, extending liquid cell microscopy to new applications, taking advantage of beam effects for nanomanufacturing such as the patterning of nanostructures, and correctly interpreting experimental observations. Additionally, our results indicate that liquid cells provide a new tool to study radiolysis effects on materials and processes.