Demonstrating Conservation of Mass with Potassium Iodide and Lead Nitrate

One of the highlights in our recent science lesson was demonstrating the conservation of mass using the reaction between potassium iodide and lead nitrate. This chemical reaction is perfect for illustrating this principle, as it creates a dramatic visual effect: when the two colourless solutions are mixed, a bright yellow precipitate of lead(II) iodide forms almost instantly. The students, especially the girls in my class, were captivated by the reaction, as its rapid, vivid result made the abstract concept of conservation of mass tangible and memorable.



The beauty of this experiment lies in its simplicity and effectiveness. Unlike reactions that produce gases, which may escape and complicate mass measurements, this reaction generates a solid product. This allows us to weigh the reactants and the final mixture to confirm that no mass is lost—just redistributed as a new compound forms.


Safety was a top priority during the experiment. Since lead compounds are toxic, we performed the reaction in a fume cupboard to protect everyone from harmful exposure. The waste, containing lead iodide, was collected carefully and disposed of according to strict environmental guidelines, ensuring that no pollutants were released into the environment.


This short, visual experiment not only demonstrated a fundamental scientific law but also sparked excitement and curiosity in the classroom. It was a clear reminder that hands-on activities can make learning engaging and impactful.

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