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Introduction Real understanding of chemistry demands the bringing together of conceptual understandings in a meaningful way. According to Johnstone and Bodnercited in Sirhan (2007), what is taught by teachers is not always what is learned by students. They added that while students show some evidence of learning and understanding in examination papers, research showed evidence of misconceptions and rote learning of certain areas of basic chemistry which are still not understood. Johnstone again indicated that the nature of chemistry concepts and the way the concepts are represented (macroscopic, microscopic, or representational) make chemistry difficult to learn. Ozkaya (2002) attributes learning difficulties in electrochemistry to a general lack of conceptual understanding and attributes this to insufficient textbook explanations of these concepts. Research has shown that students have difficulties in conceptualizing redox reactions (Osterlund, 2009). According to Sirhan (2007), numerous reports supported the view that the interplay between macroscopic and microscopic worlds is a source of difficulty for many chemistry learners. Conceptual change points to the development and transformation of students understanding from their naive conceptions to scientific explanation (Uzuntiryaki, 2003).Conceptual change theory takes constructivism as its foundation, and addresses how thoughts must be altered in order to coincide with scientific theory (Meyers, 2007).The model has direct implications regarding how to construct instruction to achieve conceptual change (Read, 2004). Chemistry instruction should be designed to present anomalies so as to create cognitive conflict. This will create a disequilibrium, which leads to dissatisfaction with the existing concept, and ultimately to a willingness to accommodate a new concept. Teaching chemistry should therefore focus on providing students with opportunities in which they have cognitive conflict and develop different structures based on their experience. Conceptual change can be accomplished if students are given opportunity to be aware of their ideas, to encounter ideas other than their own and to realize the deficiency in their reasoning. According to Balci (2014), conceptual change texts are texts designed to change students' alternative conceptions and focus on strategies to promote conceptual change by challenging students' alternative conceptions, producing dissatisfaction, followed by a correct explanation which is both understandable and plausible to the students.

The concept of hybridisation is one of the most difficult concepts for chemistry students to grasp at all levels of learning. Research showed the students conceptual difficulty ranged from their lack of the pre-requisite knowledge for grasping the topic hybridisation to chemical bond formation and orientations of atomic orbitals. This study investigated the difficulties Senior High School Students face in learning hybridisation. The study adopted a mixed-method approach using the sequential exploratory design. Purposive sampling was used to select six schools that offer elective chemistry subject. Simple random sampling was then used to select 120 Senior High School form 1 students to take part in the study. Convenient sampling was used to interview 24 students from the sample. Hybridisation Achievement Test (HAT) and Semi-structured interviews were self-constructed and used to collect data. Descriptive statistics and Content analysis were used to analyse the data. Results showed that, majority of students had difficulties in explaining the concept of hybrid orbitals, writing the electron configuration of 6C, explaining the effect of hybridisation on formation of chemical bonds in 6C, and type of hybridisation and shapes of compounds such as NH3, OF2, BCl3, CO2, SiO2, C2H2, BeF¬2 and C2H4. The study also showed that students had difficulties in explaining and demonstrating the formation of C = C double bond in ethene and other compounds. Equally students were challenged in demonstrating the differences between the formation of sigma and pi bonds in compounds. Students had difficulties in using electron orbital diagrams to explain the shape of CO2 as linear. The study concluded that Senior High School form 1 chemistry students in the Upper West Region of Ghana lacked the most basic and fundamental concept of hybridisation. Teachers need to use conceptual change instructional approaches to teach hybridisation in order to foster students’ understanding and reduce misconceptions. Keywords: atomic orbitals; hybrid orbitals; hybridisation; mixed- method approach; sigma and pi-bonds;