Previous research on students' difficulties in learning chemical equilibrium has shown that students hold misconceptions about this topic, and this often results in unintended learning. However, recent developments in human learning from the behavioural sciences perspective indicate that students' difficulties in learning are much more than misconceptions. The purpose of the study was to identify the knowledge and skills that students lack in successfully solving chemical equilibrium problems involving calculation of the equilibrium constant, K[subscript c]. Participants of this study included three students from a high-achieving school and four students from a low-achieving school in the Bohlabela district of Mpumalanga province of South Africa. Data sources included transcripts of students' think-aloud protocols, video recordings of students' problem-solving performance and students' written answers. Results showed that students at the low-achieving school applied personal algorithms exclusively at a superficial level in all phases of the problem-solving process, and committed many errors. Their main difficulty was a lack of conceptual awareness of how a chemical reaction progresses to equilibrium. On the other hand students at the high-achieving school demonstrated conceptual understanding in solving problems in familiar contexts, but lacked the skills for dealing with problems in an unfamiliar context; few errors were observed in their solutions. The results suggest that contextual knowledge is of a higher-order type, and is likely to emerge after students have acquired considerable skills in self-regulation. The implications of the results for teaching of chemical equilibrium are discussed.