We discuss how to reduce the incongruence between the outcomes (both cognitive and affective) of the conventional secondary chemistry curriculum and what is to be attained: the meaningful connection of students' learning to daily life and societal issues. This problem is addressed by a design study with one curriculum unit about "Water Quality". With several research cycles using developmental research, we developed an emergent understanding about an instructional framework for curriculum units that embodies a coherent "need-to-know" principle and is based on authentic practices. Using this framework we show with some other examples how a context-based chemistry curriculum can be constructed based on the developed "need-to-know" principle. (Contains 2 figures and 4 tables.)