Adan Hussain January 28, 2010
Pennies, as Good as New
Objective: To introduce the concept of chemical reactions and to distinguish between physical and chemical changes.
Introduction: Physical changes preserve the substance in that there is no change in the chemical makeup of the substance. Only the particles of the substance are rearranged. Examples of physical changes are melting ice, cutting paper, and molding clay. Chemical changes involve the transformation of the original substance into another substance/s; particles of the substance are broken apart and rearranged into new substances. Some examples include methane gas igniting, paper burning, milk going sour, or metal rusting. Chemical reactions are characterized by chemical change, which yield one or more substances. Chemical reactions also preserve mass and energy. In this activity, students will distinguish between a chemical change and a physical change and make educated guesses to understand the chemical reaction.
Vocabulary: Matter, Chemical Change, Physical Change, Chemical Reaction, Conservation of Mass
Materials:
Containers or anything can hold at least 500 (about 2 cups) mL of liquid.
250mL (about 1 cup) of Vinegar*
80mL (about ½ cup) of Salt*
50mL (about ¼ cup) Lemon or Lime Juice*
Pieces of Iron or Iron nails
10-20pennies (preferably rusty pennies)*
Stirrers (Popsicle sticks, spoons, etc.)
Measuring cups or beakers
*For each container
Safety: If sharp pieces of iron are used, then students must be careful handling them.
Demonstration:Basic vinegar and baking soda demonstration. Most students have seen this reaction before. Use it to talk about the chemical reaction of baking soda and vinegar. The reaction creates water, a salt (CH3COONa) and carbon dioxide, all of which are completely different from the original reactants.
Procedure:
1. Add 250mL of vinegar to container
2. Add 80mL of salt to vinegar and stir thoroughly. Make sure the salt completely dissolves into the vinegar.
3. Add 10- 20 pennies to the solution. Observe what happens.
4. Add 50mL of juice to the solution. This should make the solution more acidic and allow the copper to coat the iron faster.
5. Add a piece of iron into the solution. Make sure that a part of the metal is not in the solution so that students can notice the difference.
6. Let it sit for 20-40 minutes and observe the metal.
What to Expect:
The reaction between the pennies and the solution should be instant. The rust on the pennies will clear off. The rust is actually copper oxide, which reacts with the vinegar leaving copper ions in the solution. The salt is a catalyst. These copper ions originally left their electrons on the coin. The same reaction occurs when the iron is added. Some iron ions leave the nail leaving electrons behind. The electrons on the iron then attract the copper ions. This reaction takes some time, but there should be a noticeable difference in 20-40 minutes. If there is no change, leave the iron pieces in the solution longer. Students can do another activity while they wait. I would suggest giving students different examples changes and have them figure out if it was physical and chemical. Here are some examples: paper burning, ice melting, dry ice sublimating, clay being molded, acid rain corroding a piece of metal, lightning, and stomach acid breaking down food.
Suggestions:
Talk about what pennies are made of and why they get dirty over time before this activity. This would be a good way to introduce another chemical reaction, metal rusting.
Supplementary Exercises and Extensions:
Use other acidic liquids such as orange juice and explore why acids clean up the penny.
Use this activity to further talk about the elements on the periodic table, the elements involved in these reactions, and the chemical change that took place on the atomic level.
References
Pennies, as Good as New!
You are working for the United States Mint. They tell you that you need to make the pennies as good as new because they are too dirty. In this document, there is recipe for the top secret solution that can clean all pennies. It also has another secret….
Let’s go over some things you should know before we start
What are pennies made of?
Why do old pennies look dirty?
Recipe:
- Put 250mL of Vinegar and pour it into your container.
- Put 80mL of Salt and pour it into your container.
- Mix with your solution until all of the salt dissolves.
- One by one, add the pennies. Watch what happens!
What changed about the pennies?
Was this a chemical change or a physical change?
Where did the rust go?
- Now add 50mL of Lemon/Lime Juice.
- Place a piece of iron into the container. Make sure a part of the iron is sticking out of the solution.
- Leave it there and come back in 30 minutes.
- Observe
Did you notice any changes? What happened?
Was this a chemical change or a physical change?
Can you explain what happened?
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