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Chemistry LibreTexts

4.2: Conservation of Mass

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  • Page ID
    52748
  • If you build a campfire, you start with a big pile of logs, As the fire burns, the pile of logs slowly shrinks. By the end of the evening, all that's left is a small pile of ashes. What happened to the matter that you started with? Was it destroyed by the fire?

    Where's the Matter?

    It may seem as though burning destroys matter, but the same amount, or mass, of matter still exists after a campfire as before. Look at the sketch in the figure below. It shows that when wood burns, it combines with oxygen and changes not only to ashes but also to carbon dioxide and water vapor. The gases float off into the air, leaving behind just the ashes. Suppose you had measured the mass of the wood before it burned and the mass of the ashes after it burned. Also suppose you had been able to measure the oxygen used by the fire and the gases produced by the fire. What would you find? The total mass of matter after the fire would be the same as the total mass of matter before the fire.

    Burning is a chemical process.

    Law of Conservation of Mass

    This burning campfire example illustrates a very important law in science: the law of conservation of mass. This law states that matter cannot be created or destroyed. Even when matter goes through a physical or chemical change, the total mass of matter always remains the same. How could you show that the mass of matter remains the same when matter changes state? You could find the mass of a quantity of liquid water. Then you could freeze the water and find the mass of the ice. The mass before and after freezing would be the same, showing that mass is conserved when matter changes state.

    Example 1

    If heating 10 grams of CaCO3 produces 4.4g of CO2 and 5.6g of CaO, show that these observations are in agreement with the law of conservation of mass.

    SOLUTION

    • Mass of the reactants = 10 g
    • Mass of the products = 4.4 + 5.6g = 10 g.

    Because the mass of the reactants is equal to the mass of the products, the observations are in agreement with the law of conservation of mass.

    Summary

    Burning and other changes in matter do not destroy matter. The mass of matter is always the same before and after the changes occur. The law of conservation of mass states that matter cannot be created or destroyed.

    Explore More

    At the following URL, apply the law of conservation of mass to a scene from a Harry Potter film. Then answer the questions below.

    1. What is the mass of the professor in kilograms? What is the mass of the cat in kilograms? (Hint: 1 pound = 0.45 kilograms)

    2. The scene must be magic because it defies the law of conservation of mass. Explain why.

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.