Conservation of Mass

Ck12 Science

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Printed: August 5, 2014

AUTHORCk12 Science

www.ck12.org Chapter 1. Conservation of Mass

CHAPTER1 Conservation of Mass

• State the law of conservation of mass.

Have you ever lost a screw?

The following situation happens all too often. You have taken apart a piece of equipment to clean it up. When youput the equipment back together, somehow you have an extra screw or two. Or you find out that a screw is missingthat was a part of the original equipment. In either case, you know something is wrong. You expect to end up withthe same amount of material that you started with, not with more or less than what you had originally.

Law of Conservation of Mass

By the late 1700s, chemists accepted the definition of an element as a substance that cannot be broken down intoa simpler substance by ordinary chemical means. It was also clear that elements combine with one another toform more complex substances called compounds. The chemical and physical properties of these compounds aredifferent than the properties of the elements from which they were formed. There were questions about the detailsof these processes.

In the 1790s, a greater emphasis began to be placed on the quantitative analysis of chemical reactions. Accurate andreproducible measurements of the masses of reacting elements and the compounds they form led to the formulationof several basic laws. One of these is called the law of conservation of mass, which states that during a chemicalreaction, the total mass of the products must be equal to the total mass of the reactants. In other words, masscannot be created or destroyed during a chemical reaction, but is always conserved.

As an example, consider the reaction between silver nitrate and sodium chloride. These two compounds willdissolve in water to form silver chloride and sodium nitrate. The silver chloride does not dissolve in water, so itforms a solid that we can filter off. When we evaporate the water, we can recover the sodium nitrate formed. If wereact 58.5 grams of sodium chloride with 169.9 grams of silver nitrate, we start with 228.4 grams of materials. Afterthe reaction is complete and the materials separated, we find that we have formed 143.4 grams of silver chloride and85.0 grams of sodium nitrate, giving us a total mass of 228.4 grams for the products. So, the total mass of reactantsequals the total mass of products, a proof of the law of conservation of mass.

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Watch a video demonstrating this idea at http://www.youtube.com/watch?v=J5hM1DxaPLw

MEDIAClick image to the left for more content.

Summary

• The law of conservation of mass states that, during a chemical reaction, the total mass of the products must beequal to the total mass of the reactants.

Practice

Questions

Use the link below to answer the following questions:

https://www.etap.org/demo/grade8_science/lesson5/instruction1tutor.html

1. If you want to say something about chemical reactions, what would you use?2. What does the Law of Conservation of Mass mean?3. How much oxygen gas would I need if I react six molecules of hydrogen?4. How many molecules of water would be formed?

Review

Questions

1. State the Law of Conservation of Mass.2. What does this law mean?

• law: Accurate and reproducible measurements of the masses of reacting elements and the compounds theyform led to the formulation of several basic laws.

• conservation of mass: States that during a chemical reaction, the total mass of the products must be equal tothe total mass of the reactants.

• reactant: A substance that undergoes change in a chemical reaction.• product: The result of a chemical reaction.

References

1. Douglas Heriot. http://www.flickr.com/photos/dhdesign/3843848623/ .

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