This video starts with Lavoisier's 18th-century experiments and uses them to show why the Law of Conservation of Mass isn't just a rule to memorise — it's a consequence of the fact that atoms are never created or destroyed in a chemical reaction. You'll see why reactions that appear to gain or lose mass are really just losing gases to or gaining them from the surroundings, and you'll work through the calculation method that lets you find a missing mass from a balanced equation.

What You Will Learn

00:38What the Law of Conservation of Mass states and why it follows from the atomic nature of matter.

02:06Who Lavoisier was and how his sealed-vessel experiments provided the first strong evidence for conservation of mass.

02:43How to write and interpret word equations for chemical reactions.

06:47Why some reactions appear to lose mass when a gas escapes to the surroundings and how to explain this correctly.

07:58Why some reactions appear to gain mass when a gas from the surroundings is incorporated into the product.

09:01What thermal decomposition is and how the calcium carbonate example illustrates mass changes in open systems.

10:50How to use conservation of mass to calculate a missing mass from a reaction.

11:36How to apply conservation of mass concepts to multi-step exam questions.