RESUMEN
Studies on students' problem-solving skills worldwide suggest there is a room for improvement. This study aimed at improving upper-secondary school students' problem-solving skills in chemistry lessons. They were given a problem tasks pre-test focused on their conceptual knowledge regarding the periodic table, ability to apply knowledge on the factors affecting chemistry reaction rate and compounds' properties. Most students (72 out of 112) did not succeed to solve the tasks. For this reason, an intervention was designed based on a study using eye-tracking combined with think-aloud. It consisted of students' working on (PISA-like) context-based chemistry problem tasks with a special scaffolding. A teacher provided formative assessment promoting students' expansive strategies. The intervention's effect was again assessed using problem tasks in two post-tests. The results showed the action plan was successful in helping the majority of students reach above-average test score. The ratio of successful solvers also rose and unsuccessful significantly declined.