The purpose of science education in our country as in many western countries is to educate scientific literate people. Discussing and deciding on socio-scientific issues (SSI) was considered as an important part of scientific literacy. SSI are complex scientific and technological issues which are not clarified yet and have social consequences. 

Hydroelectric Power Plants (HPP) the systems which produces energy form the potential energy of water in the river. While most of the water is taken into a tunnel, some amount of the water which is calculated for that specific area is stayed on the river bed. This amount is calculated by experts. However, construction and operation of HPP is given to private companies for 49 years. Two thousand HPP project is planned throughout the country.

 HPP are controversial issues, because there is social debate on HPP. Villagers do not accept construction of HPP and sometimes they even prevented its construction on their site. In addition, some citizens find the number of HPP (2000) too many. It is also observed that HPP change the ecology in the region depending on less water on the river bed and division of ecological regions because of the construction. It has also scientific and technological dimension. Some authorized people do not trust the calculation of the amount of water to be left in river bed. Thus, citizens claimed that it is not calculated reliably and it is usually less than required. Since implementation of HPP is done by private companies, it is left to the company to obey this regulation and sometimes they do not. On the other hand, HPP provide renewable and cheap energy and it is argued that lots of villagers could benefit from employment.  Thus, advantages and disadvantages of HPP are a dilemma and not clear yet. Depending on these reasons, HPP were accepted as a SSI in this study.

People use some reasoning modes when they making a decision in resolving complex SSI. Eggert and Bogeholz (2009) stated that people should make a choice with comparing different solutions and options, evaluating scientific knowledge, and judging situations. According to them, making decision on SSI could not be resolved intuitively and spontaneously, instead of this, students should experience different reasoning or argumentation process. For analyzing reasoning modes, some categorization was proposed like social, ecological, economic, practical, science/technology-oriented arguments in several studies (Jimenez-Aleixandre & Pereiro-Munoz, 2002; Patronis, Potari, & Spiliotopoulou, 1999; Wu & Tsai, 2007).

Argumentation is another topic scaffolds people’s active participation into the discussion and decision making process on SSI. van Eemeren & Grootendorst (2004) defined argumentation: “Argumentation is a verbal, social and rational activity aimed at convincing a reasonable critic of the acceptability of a standpoint by putting forward a constellation of propositions justifying or refuting the proposition expressed in the standpoint”. It is argued that argumentation enables students to develop conceptual understanding, understand science as a social process, critically examine scientific claims, and be equipped in making decisions in daily life (Driver, Newton, & Osborne, 2000; Norris & Phillips, 1994). Science educators determined students’ quality of argumentation by their use of some argumentation components like claim, data, warrant, backing, qualifier, and rebuttals (Erduran & Jimenez-Aleixandre, 2008; Venville & Dawson, 2010).

Since there is a social debate on HPP in the country, it is important to understand reasoning modes and arguments of different stake holders. Thus, various people’s reasoning modes and quality of argumentation while they were making decisions about HPP were investigated in this study. This study would also inform science teacher when they take HPP into their classroom as a SSI.

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