How the heck do you teach the conservation of mass? Two components are required. First, a chemical reaction. Second, it must be easily quantifiable. In other words the mass is easy to measure and it must show very clearly that mass was not lost during the chemical reaction. I think I may have finally developed a lab activity that addresses both components.
When it comes to chemistry, electrons change places driven from around by energy chemical energy. Watch your eyes; hold your ears, and look out. In two minutes, it'll be time for another really big chemical reaction.