The purposes of this study are: to apply the ARCS model in designing an acid and bases unit, and to assess a single class of 11th graders for motivation and achievement outcomes before and after ARCS instruction. Four essential strategies for designing motivation instruction in the ARCS model were: Attention, Relevance, Confidence, and Satisfaction. We used the ARCS model in designing a 10-hour acids and bases lesson for one class of 11th graders with low interest and motivation in chemistry learning. Both the Students' Motivation toward Science Learning questionnaire (SMTSL) (Tuan, Chin & Shieh, in press) and a teacher-designed achievement test were implemented before and after instruction. In addition, students' self-reporting on time engagement in learning before and during the instruction was also collected. The results of the study indicated that both students' motivation and achievement in the acids and bases unit increased significantly (p < 0.05) after the ARCS instruction. Students' time engagement during the ARCS lessons had increased from before. Findings of the study showed that using the ARCS model to teach acids and bases unit could improve low motivated students' level of motivation and achievement. The implications for chemistry teaching will be discussed in the paper.