Law of Conservation of Mass
The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction; mass of reactants equals mass of products.
Formula
m_{\text{reactants}} = m_{\text{products}}
Definition
The law of conservation of mass says that in a chemical reaction nothing is lost and nothing is gained: the total mass of the reactants always equals the total mass of the products, because atoms are rearranged but never created or destroyed. This is exactly why chemical equations must be balanced, and the law was established by Antoine Lavoisier in the late 1700s through careful measurements made in sealed containers. Strictly, what is conserved is mass-energy rather than mass alone, since the energy released in a reaction corresponds to a mass change $\Delta m = \Delta E/c^2$ that is far too small to detect in ordinary chemistry but becomes measurable in nuclear reactions, where the law must be restated as conservation of mass-energy.
Example
Sealing baking soda and vinegar in a bag, weighing it, letting them fizz together, and weighing it again shows the same mass before and after, even though gas was produced inside. Burning $12\text{ g}$ of carbon in $32\text{ g}$ of oxygen makes exactly $44\text{ g}$ of carbon dioxide ($C + O_2 \rightarrow CO_2$), and a rusting iron nail gets heavier only because oxygen from the air has joined the metal, not because mass appeared from nowhere. Combustion of one mole of methane releases $890\text{ kJ}$, corresponding to a mass loss of $\Delta m \approx 9.9 \times 10^{-12}\text{ kg}$, about $10$ nanograms out of $80$ grams, while fission of one mole of uranium-235 loses a measurable $0.2\text{ g}$.
Key Insight
If a reaction seems to lose mass, a gas probably escaped unnoticed; if it seems to gain mass, something from the air, usually oxygen, has joined in. Before Lavoisier, chemists explained burning with an invented substance called phlogiston, but weighing metals before and after showed they gain mass when they burn, which only makes sense if something is added rather than lost. That principle now underlies every stoichiometry calculation and mass balance used to predict how much product a reaction or a factory will make before anyone runs it.