An Experimental Approach with a Twist: Helping High School Students to Understand the Concept of Limiting Reactant

High school students and students in the first years of college have difficulties in understanding the topic of stoichiometry and, in particular, the concept of limiting reactant. Proposals aimed at overcoming these learning obstacles tend to focus on analogies, and there are few sequences that approach teaching from an experimental perspective. Those that do it usually involve specific reactions whose reagents are not easily obtained or require special equipment. In this article, an experimental activity was designed to introduce the concept of limiting reactant with nonhazardous, inexpensive, and readily available materials in combination with an innovative visual and graphic representational approach. It was implemented with three groups of 16-17 years old students (N = 74) with some previous knowledge (acid and base reactions, indicators, and slight experience in handling laboratory material). From their own observations and reasoning and without the need to go through analogical mediation, it was possible to satisfactorily arrive at the concept of limiting reactant. Subsequently, the proposal was adapted for students with no prior knowledge, and it was implemented with three groups of 9-12-year-olds (N = 27) in an informal teaching environment with similar results. This intervention could potentially help high school students better understand the concept of limiting reagents and overcome future learning obstacles in relation to other chemical phenomena that rely on this concept.