The present work reports on the experimental application (after permission by the Greek Pedagogic Institute) of three lessons about the concept of molecule from a new experimental instructional material (a book) for the teaching of chemistry at the Greek eighth grade. The material aims at encouraging constructivist teaching and conceptual meaningful learning. The application was carried out by four experienced teachers in four Greek public lower-secondary schools (gymnasion), an experimental one plus three other urban ones, for two consecutive school years (2006-07and 2007-08). The pupils answered the same test twice, once before and once after the experimental teaching. It was found that student performance was considerably (and statistically significantly) improved in all schools, and was similar for both school years. The experimental group achieved considerably higher scores both in the pre- and the post-test, while the other three schools had similar achievement among them. Finally, it is notable that the new chemistry standard state school textbook was used in the second school year, but no change in achievement was noted in comparison with the previous textbook.