1. The problem statement, all variables and given/known data The conservation of Mass states that the total mass of substances is the same before and after a chemical or physical change occurs. Explain why this is true in yet not true of density. Explanations to include mathematical as well as verbal proofs. 3. The attempt at a solution The law of conservation of mass, also known as Lomonosov-Lavoisier law, states that the mass of substances in a closed system will remain constant. Mass is neither created nor destroyed. I can't explain why this is true but i think i know why it is not true of density. Density = mass/volume if a solid square has 1*1*1 volume, and mass and changes to a rectangle. According to the law of conservation of mass the mass has to be same as the solid quare that has 1*1*1* volume and mass. but density cannot area changes and there for volume changes. Different density will come out if there is diffent volume but same mass.