The main objective of this investigation was to show that a novel problem of chemical equilibrium based on a closely related sequence of items can facilitate students' conceptual understanding. Students were presented a chemical reaction in equilibrium to which a reactant was added as an external effect. A series of three studies were designed. In Study 1, the sequence of items started with a major alternative conception, namely, “After the reaction has started, the rate of the forward reaction increases with time and that of the reverse reaction decreases, until equilibrium is reached.” In Study 2, the major alternative conception was presented the last. In Study 3, instead of the sequence, only the following statement was presented: “Rate of the reverse reaction increases gradually.” In all three studies students had to agree/disagree with the statements and provide justifications. Results obtained show that at least one group of students, in Study 1 used a contradictory response pattern based on the generation and resolution of a cognitive conflict, which facilitated conceptual understanding. In Studies 2 and 3 students did not experience a similar cognitive conflict. Given the complexity of conceptual change and students' resistance to alter their alternative conceptions (cf. hard core, Lakatos )1970) Criticism and the Growth of Knowledge, Cambridge University Press, Cambridge, UK, pp. 91–106), it is suggested that changes in students' responses may have undergone a Peripheral Theory Change (Chinn and Brewer (1993) Review of Educational Research 63: 1–49).