An inquiry-based laboratory activity to determine the chemical composition of a component in alkaline detergents, sodium sesquicarbonate (SSC), is proposed. On the basis of introductory demonstrations by the instructor on the chemical properties and reactions of SSC, students propose the hypothetical composition of SSC and possible quantitative experiments to determine the chemical composition by fully utilizing their knowledge of sodium carbonate and sodium hydrogen carbonate. The composition is determined by neutralization titration and gravimetric analyses for the thermal decomposition and the reaction with acid. The usefulness of the present inquiry-based activity in a chemistry course at high school and in a general chemistry course at university or college is discussed by reviewing the results of the practical applications to students' laboratory. (Contains 1 table and 3 figures.)