The Law of Conservation of Mass: Definition, Equation & Examples

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  • 0:05 Definition of the Law…
  • 1:38 Importance
  • 2:23 Examples
  • 3:18 Equations
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Read about the law of conservation of mass, its role in chemistry, and its importance in the universe. Learn the equation, review examples, and test your knowledge of the material with a short quiz.

Definition of the Law of Conservation of Mass

Imagine you enter a closed system, a room that is perfectly sealed where nothing can enter the room and nothing can escape. In this closed system, you light a candle and let it burn, watching as some of the wax seems to disappear as the flame travels down the wick. Where does the wax go? Is it truly disappearing?

The law of conservation of mass states that in a closed system, the mass of the system cannot change over time. Look at our example of the candle in the closed room. Though much of the wax itself is no longer present in its original form, all of the mass of the wax is still present in the room, albeit in a different form.

When the flame was lit, oxygen gas from the room reacted with the candle wax to produce water vapor and carbon dioxide gas. If you massed the reactants oxygen and wax, it would equal the mass of the products water and carbon dioxide. We can remember the law of conservation of mass with this simple statement:

  • The mass of the reactants must equal the mass of the products.

Sadly for fans of magic, anything that has mass, including matter and energy, cannot be created or destroyed. That means, mass cannot simply appear out of nowhere and equally it cannot disappear. Matter may change forms however, giving the illusion of nothing out of something or vice versa, but the mass of the matter is always the same before and after the change. If 22 grams of reactants go into a chemical reaction, then 22 grams of products must be produced.


Discovery of the law of conservation of mass helped to turn chemistry into the respectable science it is today. Chemistry has its foundations in alchemy, a protoscience that put much stock into magic and mysticism. With the advent of the law of conservation of mass, chemists took the mystery and illusion of alchemy and brought predictability and reliability to the science of chemistry.

The law of conservation of mass is very important to the study and production of chemical reactions. If scientists know the quantities and identities of reactants for a particular reaction, they can predict the amounts of products that will be made. Chemical manufacturers can increase efficiency by applying the law of conservation of mass to their laboratory practices.


Imagine you are lighting up your gas grill for the first summer barbecue. The propane from your heavy gas tank reacts with the oxygen in the air, generating a hot blue flame. The products of this reaction are water vapor and carbon dioxide gas.

Chemical and word equations for combustion of propane

If you were to capture all of the water vapor and carbon dioxide produced as you grill your food, the total mass would equal that of the propane and oxygen that went into the reaction. If 100 grams of propane and oxygen are used, then 100 grams of water vapor and carbon dioxide are produced.

In another scenario, you let a 10-gram ice cube melt within a closed container on a hot day. Though the ice cube will gradually changes forms, from liquid to vapor, the mass of the container will never change. Even once completely vaporized, the mass of the water in the system will be 10 grams.

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