We show how the overall concentration distribution of the product in a randomly stirred reactive mixture is modified by the chemical reaction. We decipher the intermingled effects of stretching enhanced diffusion and chemical production on the evolution of each concentration level, and show why chemical kinetics contributes to a “rectification” mechanism when low concentration levels grow faster than higher ones. The rectification amounts to a compression of the product concentration distribution, which is found to be narrower around its mean than for a pure, nonreactive scalar. This phenomenon is illustrated through a series of quantitative experiments in which scalar mixing and the effects of the reaction are measured concomitantly, but independently.

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• Chemical reactions
• Pattern formation
• Reacting flows
• Reaction diffusion systems
• Hele-Shaw cell