Education insights.

Practical work is the most expensive and most widely mandated component of secondary chemistry teaching, yet four decades of research have failed to establish a stable relationship between handling substances and understanding chemical ideas. This article argues that the difficulty lies less in the quantity or authenticity of laboratory activity than in the epistemic work that tasks require students to perform. A critical narrative review of research on school laboratories, guidance studies, and chemistry-specific work on representation is used to develop a framework of five epistemic functions: observing with purpose, modelling and explaining, coordinating representations, warranting claims, and deciding under uncertainty. The framework treats a practical task as a demand profile rather than a label, so that two activities with identical apparatus can make very different intellectual demands. Evidence is mapped onto the functions, showing that observational and mechanistic aims are comparatively well supported, whereas the justification of claims and the transfer of evidence to decisions remain weakly supported and rarely assessed. The article specifies a scaffold architecture that makes the epistemic object of a lesson explicit without reducing it to a recipe, and illustrates the argument with a two-lesson investigation of the iodine–persulfate clock reaction, for which task sequences, a representational map, an illustrative data set, and an analytic rubric are provided. The educational value of practical work, it concludes, depends on whether its tasks require students to produce and defend chemical claims.