Liquid-phase transmission electron microscopy has been used to study a wide range of chemical processes, where its unique combination of spatial and temporal resolution provides countless insights into the nanoscale reaction dynamics. However, achieving sub-nanometer resolution has proved difficult due to limitations in the current liquid cell design. Here we present a novel experimental platform for in situ mixing using a specially developed two-dimensional heterostructure-based liquid cell. The technique facilitates in situ atomic resolution imaging and elemental analysis, with mixing achieved within the immediate viewing area via controllable nanofracture of an atomically thin separation membrane. We use this novel technique to investigate the time evolution of calcium carbonate synthesis, from the earliest stages of nanodroplet precursors to crystalline calcite in a single experiment. Our observations provide the first direct visual confirmation of the recently developed liquid-liquid phase separation theory, while the technological advancements open an avenue for many other studies of early stage solution-phase reactions of great interest for both the exploration of fundamental science and developing applications.