We applied cognitive load theory in an heuristic out-of-school science lesson. The lesson comprises experiments concerning major attributes of NaCl and was designed for 5th to 8th grade students. Our interest focused on whether cognitive load theory provides sufficient guidelines for instructional design in the field of heuristic science education. We extracted student clusters derived from pre-knowledge and learning success. We characterised students, based on cognitive achievement, mental effort, and instructional efficiency. Cluster analyses revealed three student clusters with quite satisfying results. Two further clusters showed improvable results, two showed no learning success, which may point to difficulties in coping with the learning setting. Motivational characterisation will refine the results, and may confirm starting points to advance cognitive load theory in heuristic science education.