Summary

In this lesson, students will develop a better understanding of physical and chemical properties of matter by comparing the composition of different pennies. This is done by determining the density of different pennies which will be compared to the density of different metals.

Grade Level

Middle and High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lesson, students should be able to

• Identify and differentiate between physical and chemical properties.
• Define alloys.
• Identify how mixtures can affect physical properties.

Chemistry Topics

This lesson supports students’ understanding of

• Density
• Physical Properties
• Chemical Properties
• Physical Change
• Chemical Change
• Alloys

Time

Teacher Preparation: 25 minutes

Lesson: 1-2 hours

Materials (per group)

Experiment 1: Golden Penny

• 3 pre-82 pennies
  • You can use post-82 pennies but be careful since the zinc core has a lower melting point than the copper penny
• 1M zinc chloride solution, 25 mL
• Zn, granular or mossy, 2.0-2.2 g
• Distilled or deionized water, for rinsing pennies
• 150-mL beaker
• 2, 250-mL beakers
• 25-mL graduated cylinder
• Balance, 0.1 g
• Hot plate
• Watch Glass
• Tongs

Experiment 2: Density of Pennies

• 25 pre-82 pennies
• 25 post-82 pennies
  • (Can collect pennies from students to use in the lab if you have a shortage of pennies)
• 0.01 g balance
• 50.0 mL graduated cylinder
• water

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
• Zinc chloride solution can damage skin. If any of it gets on your skin, immediately wash the affected area with cold tap water.
• Zinc dust may be very flammable. It can spontaneously combust when in contact with moisture.

Teacher Notes

• Background: This lesson could be used to introduce physical and chemical properties of matter or to reinforce the students understanding of how to describe matter by using physical and chemical properties. Background information for a teacher that may be helpful:
  • Physical and chemical properties and changes video (5 minutes, 42 seconds)
  • Physical and chemical properties explained video (10 minutes 33 seconds)
• Pre-1982 penny composition is 95% copper and 5% zinc. Post-1982 penny composition is 97% zinc with a thin coating of copper. The melting point of copper is 1083˚C , and the melting point of zinc is 419˚C. Thus, the melting point of a post-82 penny will be much lower than a pre-82 penny.
• Density can be used to help identify which metals are found in the pennies.
• Engage: Give students the article “The Captivating Chemistry of Coins” to read and answer the engagement questions on the reading worksheet. This can be done as a homework assignment or on day 1 of the lesson.
• Explore: (10-15 minutes)
• Students share their responses to the reading questions in groups of 2-3. Then have each group share a summary of their findings with the class. Students should come up with the definition of alloy, notice how the compositions of the coins differ and may wonder how countries decide on composition of coins.
• Next, the teacher should define physical and chemical properties of matter. Introduce alloys and how properties of matter may change given the composition of the alloy. For example, density or melting point will be different than those of the pure substance.
• Experiment: Golden Penny (35 minutes)
• The experiment involves turning a copper penny into a silver colored penny (zinc coating the penny) and then a golden penny (zinc-copper alloy). On appearance, the students will think that a chemical change has occurred but in reality, it is a physical change. When the penny is treated in the zinc-zinc chloride solution, a layer of zinc is coating the penny. When the penny is heated on the hot plate, the zinc and copper atoms mix to form a zinc-copper alloy which is golden in color.
• Tips for the experiment:
  • The pennies need to be clean and dry before use. To clean the pennies, drop them into a salt-vinegar solution (2-3 g NaCl/15 mL vinegar) for about 30-60 seconds.
  • Prepare 1M zinc chloride solution (68.1 g zinc chloride (s) + 1 mL 12M HCl/ 500 mL solution).
  • Zinc dust can be very flammable. It can spontaneously combust when in contact with moisture. Please refer to the SDS for specific information.
  • Disposal of the zinc-zinc chloride solution: Collect all the used solutions from each lab group. Separate the zinc solid from the solution. If the sinks are connected to a sanitary sewer system and water treatment plant operating with an effluent, the zinc chloride solution may be discarded in the drain and flushing with ten-fold water. Zinc metal may be disposed of in landfill. If the sinks are not connected to a sanitary sewer system and water treatment plant, disposal with landfill waste by wrapping material into newspaper/cardboard. Always check with your local waste disposal site to see what materials are acceptable for disposal at the site.
  • In the experiment, when the penny is placed in the zinc-zinc chloride solution, the penny will turn silver. The silver color penny is a zinc-coated penny. In the zinc-zinc chloride solution, the zinc ions are reduced (by gaining electrons) and deposit zinc atoms onto the penny.
  • In the experiment, when the silver colored penny is heated, the zinc and copper atoms will mix, turning the penny golden. When heating the zinc-coated penny, the zinc and copper atoms mix to form a golden color brass alloy.
  • Brass is an alloy of zinc and copper. The difference in color is dependent on the percent of zinc to copper found in the composition of the alloy.
• Further experimentation: Use Density of Pennies handout:
  • This purpose of this further experimentation is for students to test if the treated pennies were made of the same substances or if a new substance was made.
  • One method is for the students to do the mustard test referred to in the article.
  • Alternatively, the students can use density of the pennies. This is done by measuring the mass and volume of different number of pennies. You can collect pennies from students and have them sort out into two groups-pre-1982 and post-1983 pennies. After the students have completed this activity they can design an experiment to help them determine if the composition of the different pennies is copper, silver and gold. Comparing the density of the different pennies to the densities of the different metals. Density of copper is 8.96 g/mL; density of silver is 10.5 g/mL; density of gold is 19.3 g/mL. The accepted density of brass is 8.5 g/mL.
  • Note that that it is difficult to make measurements on one single penny using this method so we are using a larger sampling in order to get more data values that can be used to determine an accurate value.
• Explain: (10 minutes) During this time review the results from the experiment. Have students reflect on the following:
  • Was something new made? How can you tell? What further testing can be done?
  • Potential student responses:
    • Students may come up with a variety of answers, such as no new element was made because we cannot make elements. Or they may answer, yes, it is possible.
    • Allow students to explore options. Further testing can include but not necessarily limited to testing with mustard on the coins to see if silver coin turns black, determine the density of the penny by calculating the mass/ volume ratio, test melting points, etc.
  • Allow students to provide evidence for their explanations. For homework, ask students to brainstorm how they can use the differences in properties to test if the composition of the pennies were copper, silver and gold. A variety of options are acceptable as long as there is some explanation on why the students chose the test.