Argumentation has a pivotal role in science activities because scientists produce scientific knowledge through it (Kuhn, 1993), also science can be defined as a social activity that holds argumentation in its centre (Zohar & Nemet, 2002). Besides, engaging argumentation is an essential component of scientific literacy (Driver, Newton & Osborne, 2000; Jimenez-Aleixandre & Erduran, 2008; NRC, 1996, 2012). Scientifically literate individuals actively engage in argumentation to make critical decisions in science-related social issues (Driver et al., 2000; Kuhn, 1993; Wan & Wong, 2013). Also, recently done fundamental changes in science education area emphasize the importance of students' argumentation in science education and suggest encouraging students to make more arguments (MONE, 2013, 2018; NRC, 1996, 2012). Ensuring scientific argumentation in science classrooms develops students' abilities in constructing and analyzing arguments which they need it in their future life, and it also provides students discover the nature of science (Bell & Linn, 2000; Driver et al., 2000; McDonald, 2010; Sandoval & Milwood, 2008).

To keeping up the change in science and technology area, a curriculum reform was done on 1-8 grades Turkish science education curriculum in 2005. The new curriculum was designed based on the constructivist approach by the Ministry of National Education of Turkey (MONE, 2005).  After a revision was done in 2013 depending on the previous curriculum's general frame, the student-centred approach has been adopted, and students' active constructive role through gathering knowledge has been protected.  As stated by the revisionstudents start to take science courses from 3^rd grade, and the place of argumentation was emphasized in the inquiry-based approach, for example, it was mentioned in the curriculum that;

Teachers should ensure that their students are involved in dialogues to express their ideas freely, support their ideas for different reasons, and develop opposing arguments to refute their friends' claims. In written or oral discussions involving contradictory arguments, teachers should guide their students' arguments based on valid data (MONE, 2013, p.3).

The current science curriculum in Turkey aims to develop argumentation skills of the students. Accordingly, the purpose of the present study is to visualizestudents' argumentation and analyze them in terms of their structure -quality- and scientific credibility.

This study was conducted with 8th-grade students enrolled in a state middle school science course in Turkey. The school was selected through convenience sampling. Also, 8th-grade students were intentionally preferred considering Piaget's cognitive development stages (Inhelder & Piaget, 1958). According to Piaget, the fourth and the last stage of cognitive development is the Formal Operational Stage, it begins at approximately 11 to 12 years of age and continues throughout adulthood. At this stage, people get hypothetico-deductive thinking which is also essential to make arguments. Therefore, the study participants were selected from the oldest students in the middle school educated at 8th grade. The mean age of the participants was 14.10 in the present study. A total of 109 8th grade students participated in the study. The data collection instrument was prepared for the current study. The instrument includes a case and flowing questions that enable students to create arguments over the case; therefore, they comprise an open-ended scientific issue. The case was designed by an expert on science education (also an in-service science teacher) and the current study researcher. The subject of the case prepared related to Living Things and Energy Relations unit of the current science curriculum. According to Puvirajah's (2007), the SASC (Structure of Argumentation and Scientific Credibility) model, the students' arguments in the current study were examined. In the present study, students' arguments were based on scientific issues, and investigating only the arguments' quality was not sufficient, so the arguments' scientific validity was also investigated. The SASC model serves the purpose of the current study since it was designed to analyze both structural quality and scientific credibility of an argument. It has two core components; namely, the structure of arguments and scientific credibility arguments.

Students asserted two arguments for the case as supporting and opposing arguments. The distribution of evidence and explanation types for the case will be presented in a Table. The supporting arguments are more than the opposing argument's number, but the difference is not so big. Students mostly preferred referential evidence and explicit scientific theory explanations in both their supporting and opposing arguments. The least preferred evidence type is provisional, and explanation type is established science authority in the supporting argument. Experiential evidence and established science authority explanation were the least used types for opposing arguments. Structural qualities of students' arguments revealed that most of the supporting arguments were at basic to proficient level and the least of them at proficient to basic level. For opposing arguments, most of them were at insufficient to basic level and again least of them at proficient to advanced level. Scientific credibility scores of students' arguments indicated that most of the supporting argument's components relate to each other and the claims. However, only evidence or explanations were related to the claims in opposing arguments. No more evidence made a claim stronger in most supporting arguments; however, only some evidence was presented in most opposing arguments. Components of the most supporting arguments were valid. On the other hand, most of the opposing arguments' evidence was probable or trustworthy.

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