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This paper summarizes the experience gained at Syngenta and at University of Aberdeen on visualization of gas-liquid mass transfer in agitated vessels. The aim is to show how simple experiments can be used to directly visualize the effect of vessel size, air flow rate, and agitation speed on the rate of mass transfer for gas-liquid reactions without the need for sophisticated probes or indirect calculations. The reaction used for visualization purposes is the "blue bottle" reaction, that is, the oxidation by oxygen of leucomethylene blue to methylene blue. This reaction produces a distinctive blue color, and its rate depends on the rate of oxygen mass transfer from the gas to the liquid phase. Therefore, this reaction can be used to visualize the rate of mass transfer and how this rate is affected by geometrical and operating parameters of agitated vessels. The described demonstration is furthermore highly convenient for educational purposes. This is the first demonstration visually illustrating scale-up effects in chemical processes. [ABSTRACT FROM AUTHOR]