The concept of relative density is introduced in the middle school and high school curricula by an intuitive approach based on the phenomena of immersion and flotation. In high school, the same concept is taught as the ratio between the masses of two bodies of the same volume. This dichotomy of the concept of relative density between the two educational cycles, separately approached, induces initial representations that can constitute epistemological obstacles for its conceptualization and scientific understanding. This study proposes an interdisciplinary approach between chemistry, physics, and mathematics, combining "intuitive understanding" and "mathematical formalism", with the aim of evolving intuitive perceptions toward a scientific understanding of the concept of relative density, based on the sorting of plastic waste. After establishing the conditions of buoyancy, immersion, and sinking, this approach highlights the existence of several types of plastics and their relative density based selective sorting. This experimentation on the buoyancy of solutions on each other and of solids in solutions provides chemistry and physics teachers with an experimental didactic tool to support their interventions in the teaching/learning process of the concept of relative density. This didactic tool has been proven fruitful for a better understanding of the concept of relative density.