In science classes, students usually learn about the law of definite proportions by the oxidation of copper. However, common procedures usually do not lead to proper results. This leads to confusion among the students because their experimental results do not fit to the theoretical values. Therefore, we invented a new procedure for this experiment to improve the introduction of the law of definite proportions. The new procedure can also be done by the students, and it is done within 15 min and provides theoretical data written in the textbook. To evaluate this improved procedure, we let 8th year students (n = 133) do the copper oxidation experiment in the traditional and the improved way. The oxidation ratio doing the improved method was 98%, whereas the traditional way was only 55%. Students' individual feedback indicates that the new method might be able to improve the understanding of the law of definite proportions.