Law of Conservation of Mass
Mass is neither created nor destroyed in chemical reactions.
About this topic
The law of conservation of mass, established by Antoine Lavoisier in the 1780s, states that matter is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products. It is the reason chemical equations must be balanced.
Atoms are rearranged during a reaction but their number and types remain the same, so mass stays constant. Mass seems to vanish only when a gas escapes, as when a fizzing tablet drops in an open flask, or to increase when a gas is taken from the air, as when magnesium burns and gains oxygen. In a closed system, the total mass measured before and after matches.
Example: burning 12 g of carbon with 32 g of oxygen produces 44 g of carbon dioxide. Lavoisier showed that when mercury is heated in air, it gains mass equal to the oxygen lost from the air, overturning the old phlogiston theory. Wood that burns to leave light ash also obeys the law, since the missing mass leaves as carbon dioxide and water vapour.
The law underpins balancing equations, stoichiometry and the idea of recycling. A misconception is that burning destroys matter. At nuclear scales, small amounts of mass convert to energy, so the more complete law is conservation of mass-energy.
Key ideas
- Lavoisier's experiments
- Total mass of reactants equals products
- Why equations balance
Quick check
Use cases: where Law of Conservation of Mass helps
- Homework: review the key idea behind Law of Conservation of Mass before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Law of Conservation of Mass without notes.
- Visual learning: play with the interactive lab to see how Law of Conservation of Mass changes when you adjust its inputs.
- Teaching a friend: explain Law of Conservation of Mass in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Law of Conservation of Mass?
Mass is neither created nor destroyed in chemical reactions.
What will I learn in Law of Conservation of Mass?
Lavoisier's experiments. Total mass of reactants equals products. Why equations balance.
Is Law of Conservation of Mass part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Law of Conservation of Mass lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Law of Conservation of Mass?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.