When Correct Labels Are Not Enough: Referents and Criteria in Students’ Classification of Matter

This is the authors' abstract. We don't add key points for this paper.

Classifying matter is foundational in school chemistry, but a correct category label does not show that a student used the criterion behind it. This study examined criteria-based understanding in matter classification, conceptualized as identifying what is being classified, selecting the relevant criterion, and using the available evidence to justify the classification. A 20-item diagnostic questionnaire─open definitions, multiple-choice classifications, explanation prompts, and particle-diagram tasks─was administered to 234 Croatian grade 7–8 students. Semistructured interviews provided response-process evidence, while analysis of the three Ministry-approved grade 7 chemistry textbooks situated the findings in their curricular context. Correct open definitions were rare, ranging from 3.0% for substance to 18.4% for compound. Multiple-choice and particle-diagram performance was higher but remained moderate. Success was not stable across task formats: no student succeeded across all three formats probing substance, and only 1.3% did so across the four formats probing compound. Substance was the weakest area, with students often not identifying the sample-level referent. At the compound–mixture boundary, only 11.1% gave a correct explanation despite higher classification accuracy. In the analyzed textbooks, the criteria underlying these classifications were often left implicit. These findings support assessing and teaching early matter classification beyond label recognition, with students explicitly identifying what is being classified, which criterion applies, and what evidence in the representation supports the classification.