Students’ attitudes, abilities and experiences influence their learning. Thus, as Ausubel (1968) stated that students’ preexisting knowledge is the one of the most important factors that affecting students’ learning. Shapiro (2004) mentioned that students’ existing knowledge is inadequate and students could not understand the concepts adequately. Shapiro also stated that students’ these understanding are also different than the scientific understanding and scientific knowledge. When student’ understanding of one concept is different from scientific understanding, it is stated that that student has a misconception. Misconception does not mean lack of knowledge and wrong description. Misconceptions appear at the end of the pre-existing knowledge and experiences related to that concept. Students having misconceptions could not make meaningful connections related to concepts and so it is difficult to eliminate misconceptions with traditional method. Because of misconceptions, students’ learning get difficult and so meaningful learning could not be occurred. Therefore, in order to eliminate students’ misconceptions and establish conceptual understanding, it is necessary for students to construct their knowledge. In order to use case-based learning in the classroom, teachers should firstly learn constructivist approach and its applications in the education faculties. Most of the courses in the education faculties are related to the properties of constructivist approach and its importance on learning. Also in some courses, in their own classroom with their peer or in real classroom with real students, pre-service teachers are doing microteaching in line with constructivist approach. However, other than education courses in education faculties, especially in content knowledge courses instructors generally use traditional approaches. However, it is important to use constructivist approach in these courses like chemistry, physics, and mathematics. The reason of this is that pre-service teachers could construct their knowledge and also the instructor could be the role model for pre-service teachers. Thus, the present is conducted on science pre-service teachers in general chemistry course. According to the most of the students, chemistry is considered as abstract and most students stated that it is difficult to understand chemistry concepts. One of the topics that students and pre-service teachers’ having misconceptions and difficult to understand were acid and bases topic. The other reason of studying with acid-base topic is that this topic is abstract, it is the one of the basic chemistry topics, it is related to the other chemistry topics (for example, matter, chemical equilibrium, chemical reactions, solubility). In addition to this, it is stated that the studies related to the development of conceptual understanding on acids and bases were generally conducted on secondary and university students and there is not much study on pre-service teachers. Thus, the participants of the study were freshmen pre-service science teachers. It is important to eliminate pre-service teachers’ misconceptions on acid and bases because pre-service teachers make meaningful learning and their future students understand this topic and future students could have lesser misconceptions. Thus, in the present study, pre-service teachers’ misconceptions related to acid and bases were eliminated by case-based learning and the development of knowledge in pre-service teachers was investigated deeply. The purpose of this study is to investigate the effect of case stories on pre-service teachers’ conceptual understanding. One of the strategies for developing conceptual understanding is constructivist approach. Case stories were prepared according to this approach. According to this approach, pre-service teachers were active and they construct their knowledge. Instructors guide the pre-service teachers. Constructivist approach covers the nature of science and how learning was implemented in the class. According to this approach, students construct their knowledge in to existing knowledge and this information is meaningful for him or her. Thus, students do not transfer the knowledge instead they construct the knowledge. There are some teaching methods that are using constructivist approach; one of them is case-based learning instruction. According to case-based learning instruction, students were active in the class; they have the responsibility for constructing their own knowledge. The role of teacher is to guide students while using cases. Case stories were prepared according to the constructivist approach and so pre-service teachers construct their knowledge. Case stories were presented to pre-service teachers as Herreid (1994) stated. Case stories were stories with a message and pre-service teachers analyzed the cases for considering the solutions the cases. Herreid (1994) stated that case-based learning could be implemented with four types. These are; lecturing, individual assignment, discussion and small group discussion. All four types of case-based learning, the properties of the cases were the same but the roles of teacher and students are different. Thus, in the present study, Herreid (1994)’s small group discussion was conducted and instructor and pre-service teachers work together and pre-service teachers analyzed the case stories with each other and they have control for analyzing the cases. In the present study, pre-service teachers were instructed with small group format because they could learn more from each other. This study was conducted on acid-base concept because many studies stated that this concept is difficult (Sheppard, 2006). In addition, acid-base concepts were abstract and this concept was one of the fundamental concepts of the chemistry and also this concept was related to the other concepts. This study is a qualitative study and this is a case study. In this study, pre-service teachers’ responses on case stories were examined deeply and they were analyzed holistically (Yıldırım, Şimşek, 2011). Therefore, pre-service teachers’ development of conceptual understanding was determined every week. The participants of the study were 72 (65 female, 7 male) first grade pre-service teachers. The mean age of the participants was 18. All participants were attending General Chemistry course and General Chemistry Laboratory course regularly. In laboratory course, pre-service teachers were applying the concepts learned in General Chemistry course. This study lasted for six weeks and pre-service teachers were presented with four case stories. Pre-service teachers were instructed with small group discussion every week in the laboratory. Pre-service teachers formed groups of four people in the laboratory. The responses of pre-service teachers to case stories were examined by the rubric developed by the researcher. While developing the rubric, all of the responses of the pre-service teachers were examined and then they were categorized as incorrect and correct answers. After that their correct answers were classified as whether they were related to the daily life and scientific theory. After this rubric was developed, two researchers other the author of this paper, analyzed the rubric and then they were get in agreement. After that pre-service teachers’ responses were coded in line with this rubric. Three of them were analyzed with these two researcher and we came in to the consensus and then all of the responses to the case stories were coded by the author of this paper.   The mean scores of each group were becoming higher and the percentage of incorrect answers to the questions at the case stories was higher at the beginning of the intervention. However, at the fourth case story, pre-service teachers’ answers to the story problems were including the scientific theories and concepts. After four week, the responses of each groups’ story problems were increasing in terms of the numbers of scientific theories and concepts and these could be show that the conceptual understanding of pre-service teachers were increasing. According to the result of the study, the average scores of the every group were increasing and pre-service teachers used scientific knowledge and theories and these shows that pre-service teachers’ conceptual understanding was increasing over the weekend. The result of the study demonstrated that case stories and case-based learning instruction were helpful for the construction of knowledge, elimination of misconceptions and pre-service teachers used scientific theories in their answers. Most of the studies related to case-based learning were generally quantitative and different than these studies; the present study is qualitative study. In the present study, pre-service teachers’ responses while answering the questions at the end of the case stories. The pre-service teachers’ responses were analyzed deeply in order to examine the development of pre-service teachers’ knowledge. Thus, the present study is important for investigating deeply the development of knowledge and for how this development is occurring. However, this study is a qualitative study and so the present study result cannot be generalized to the population. The purpose of this study is only to deeply examine the pre-service teachers’ conceptual understanding in case-based learning instruction. At the end of the study, pre-service teachers were active in the learning process. Also, they like the case stories and they like to do lesson with this method. They also mentioned that abstract chemistry concepts were become concrete with that method and they were easily make linkage between chemistry concepts and daily life. They mentioned that they were very excited to observe the daily life experiences. In sum, it could be stated that case stories were helpful for the development of conceptual understanding of chemistry concepts. Also, this study results were important for development of daily life experiences were before the development of the scientific theories and concepts. Therefore, it is suggested that in science education departments, case stories should be used in General Chemistry courses. Bu çalışmanın amacı, Fen Bilgisi öğretmen adaylarının kimya konularındaki kavramsal anlamalarını örnek olaylarla geliştirmektir. Ayrıca, çalışmada öğretmen adaylarının örnek olaylara verdikleri cevaplar üzerinden kavramsal anlamaları hakkında bir değerlendirme yapılmıştır. Bu çalışma nitel bir çalışmadır. Öğretmen adaylarına bazı kimya konuları ile ilgili örnek olaylar sunulmuş ve onlardan cevaplarını belirtmeleri istenmiştir. Çalışma 72 birinci sınıf Fen Bilgisi Öğretmen adayı üzerinde gerçekleştirilmiştir ve altı hafta sürmüştür. Öğretmen adaylarının kavramsal anlamalarında meydana gelen gelişim onların örnek olaylara verdikleri cevaplarla belirlenmiştir. Çalışmanın sonucunda öğretmen adaylarının örnek olaylara verdikleri cevapların haftalar geçtikçe bilimsel anlamda geliştiği, derslere daha aktif olarak katıldıkları görülmüştür. Öğretmen adaylarına dersi nasıl anlatılmasını istedikleri sorulduğunda pek çoğu örnek olay yöntemini sevdiğini belirtmiş, sebebi olarak da öğrendikleri somut kimya konularının günlük hayattaki karşılıklarını görmenin heyecanlı olduğunu belirtmişlerdir. Sonuç olarak, örnek olay yöntemi öğretmen adaylarının asit-baz konusunun kavramsal anlaması üzerinde etkili olduğu gözlemlenmiştir. Bu sonuçlarla, Fen Bilgisi Öğretmenliği programlarında okutulan Genel Kimya derslerinde bu yöntemin uygulanabileceği önerilmektedir.