Gas-liquid flows in bubble column reactors are inherently unsteady. In the present work, we have characterized dynamics of gas-liquid flow in rectangular bubble columns and studied the role of unsteady flow structures in liquid phase mixing using experiments and CFD simulations. The need for considering the unsteady nature of gas-liquid flows was demonstrated by using mixing simulations carried out with transient and time-averaged flow. A new methodology of using a computationally amenable 'multiple snapshots' approach, which can adequately account for dynamics of underlying flow, is proposed. CFD simulations were carried out to study the influence of superficial gas velocity and height-to-width ratio on mixing time. The predictions were compared with mixing time measured using conductivity probes. The present work gives useful insights into the mixing process in inherently unsteady flows.