The process of construction of meanings for the concepts of concentration and neutralization is explored in terms of contextualizing practices (Lemke, 1990, Talking Science. Language, Learning and Values, Norwood, NJ: Ablex) creation of meanings through connections established among actions and their context. This notion is expanded to include the connections established among concepts and their context of use, a solving problem task in a laboratory. The purpose is to document the process of meaning construction for these concepts and their transformation from mere terms into decisions and practical actions in a chemistry laboratory. We sought to combine this analysis with an epistemological focus, examining the relative epistemic status of the contextualizing practices. The conversations and actions of four grade 10 students and their teacher (second author) were recorded while solving an open task: to find the concentration of an HCl solution. The results show a progression in the process of contextualization, from an initial inability to use the concepts as part of the resources to complete the titration task, to the transformation of definitions into shared meaningful concepts that allow to take actions, combining theoretical resources with physical ones to solve the problem. A frame for categorizing contextualizing practices across epistemic levels is proposed and applied to the data. © 2006 Wiley Periodicals, Inc. Sci Ed90:707–733, 2006