Unit B Lesson 13: The Law of Conservation of Mass

Unit B Lesson 13: The Law of Conservation of Mass

Learning Targets

At the end of this inquiry, you should be able to answer the following questions:

• What happens to products of a chemical reaction in a closed system?
• What happens to products of a chemical reaction in an open system?
• What is the law of the Conservation of Mass?

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Periodic Table links:

 

Introduction

Many chemical reactions produce a gas as one of the products.  Have you ever mixed baking soda and vinegar and witnessed the violent bubbling and fizzing?  Check out the spectacular results on the "Celebrate Chemistry" video below.

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The Law of Conservation of Mass

The law states that mass cannot be created or destroyed. This means that the mass of the reactants must always equal the mass of the products.

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Do all chemical reactions follow the Law of Conservation of Mass? 

An open system freely exchanges energy and matter with its surroundings.  For instance, when you are cooking beef stew in an open saucepan on a stove, matter and energy are being transferred to the surroundings through steam.

The saucepan is an open system because it allows for the transfer of matter (adding spices in the saucepan) and for the transfer of energy (heating the saucepan and allowing heat to leave the saucepan).

Putting a lid on the saucepan makes the saucepan a closed system. A closed system exchanges only energy with its surroundings, not matter. By putting a lid on the saucepan, matter can no longer exit.  The lid prevents matter from entering and leaving the saucepan.

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Do chemical reactions that occur in closed and open systems follow the Law of Conservation of Mass?

In a closed system , the reactants and products are trapped.  No additional material may be added.  No substance can be removed.  The blue bottle experiment is an example of a reaction completed in a closed system.   It is easy to see that a reaction follows the law of conservation of mass in a closed system because none of the products is able to exit.  Does a reaction still follow the law of conservation of mass if the products are able to escape?

An open system can exchange matter and energy with its surroundings.  When an experiment is performed in an open system, products can escape into the atmosphere. 
When a log is burned, carbon dioxide gas and water vapour escape into the surrounding air. 

Although mass and energy escape into the surrounding, the mass of the products is still equal to the mass of the reactions.  Reactions in an open system still follow the Law of Conservation of Mass:  mass (matter) cannot be created or destroyed.

In the blue bottle experiment, water, glucose, potassium hydroxide, and methylene blue are mixed in a flask (figure 1). When the bottle is shaken, this mixture reacts with the oxygen in the air contained in the bottle (figure 2). What is happening here?

When the bottle is shaken, a chemical reaction occurs when the oxygen in the bottle reacts with the other chemicals. When shaking stops, the mixture goes back to clear because no more oxygen is reacting. Not only is this a closed system, but the reaction occurring in the bottle goes back and forth depending on whether the bottle is shaken or not.

Watch

 Watch "The Blue Bottle Experiment" for a demonstration and detailed explanation about this closed system chemical reaction.

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Watch

Watch the videos "Law of Conservation of Mass: Closed System" and "Law of Conservation of Mass: Open System" that describe clearly how the law of conservation of mass applies in open and closed systems.

 

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Interactive

Conservation of Mass

1. Watch the BrainPOP video to ensure that you understand the "Law of Conservation of Mass" fully. Click here to watch.
   
   You will need a username and password to access the video:
   
   • Username: 0099
   • Password: students
     
     
2. Complete the BrainPOP quiz on the Law of Conservation of Mass. Remember to check your answers at the end!
   
   Click here to take the quiz.

Watch

Review what you've learned this lesson by watching "Vacation or Conservation (Of Mass)".

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Complete the following three examples on the law of conservation of mass.  Click on the icon to reveal the answer.

Example 1

106 grams of ammonium chloride break down into 34 grams of ammonia and an unknown amount of hydrochloric acid.  How many grams of hydrochloric acid will be produced?

 

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Example 2

27 g of aluminium is added to 60 grams of hydrofluoric acid.  A certain amount of aluminium fluoride as well as 3 grams of hydrogen gas is produced.  What is the mass of aluminium fluoride?

 

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Example 3

22 g of carbon dioxide and 18 grams of water are produced when 8 grams of methane is burned.  How many grams of oxygen are required to react with methane?

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