This chapter describes the procedure for observing what happens to the mass of reactants and products in a reaction. This real-world science experiment is structured as a problem-solving activity thus helping KS3 science teachers to engage students effectively in the secondary school classroom. First, background science knowledge for this experiment is provided, along with the learning objectives, list of materials needed for the experiment (potassium iodide solution and lead nitrate solution), safety and technical notes, and a list of engaging activities to be performed by students. Data collection ideas are subsequently provided, together with useful questions to ask the students and also suggestions for homework.