Mass is a measure of the amount of matter in an object, and it is measured with a balance. In a chemical reaction the atoms of the reactants are rearranged into new products, but no atom is created or destroyed. The total number of each type of atom stays the same, so the total mass stays the same. This is the law of conservation of mass. A sealed bag of baking soda and vinegar keeps its mass on the scale; an open cup seems to lose mass only because the carbon dioxide gas leaves and takes its mass with it.