Chemical reactions are often accompanied by formation of a precipitate, evolution of gas, change in color, or pronounced temperature change. In this activity, you will observe these characteristics of chemical reactions. Enjoy the variety!

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To recognize that change of state, change in color, formation of a precipitate, or the evolution of heat are associated with a chemical change; to study reactions of copper.

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Wear protective glasses and an apron at all times. Avoid skin contact with solids and solutions. Dispose of all solutions in the containers provided by your teacher. Wash your hands before leaving the laboratory.

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First, prepare a data table. Record what you see throughout the laboratory activity.

1. Your teacher will demonstrate for the class the reaction of metallic copper with concentrated nitric acid, HNO₃. This must be done in the fume hood. (Caution: Avoid breathing poisonous gases. Avoid contact of skin with nitric acid. It burns skin and clothing.) One student will be asked to feel the side of the demonstration beaker and report to the group. The products formed are copper(II) nitrate solution, Cu(NO₃)₂(aq), and nitrogen dioxide gas, NO₂(g).
2. A solution of copper(II) nitrate was prepared earlier. One of your thin-stem pipets contains this solution. Transfer all of the solution to your labeled test tube.
3. Another thin stem pipet contains sodium hydroxide solution, NaOH(aq). (Caution: Avoid contact with sodium hydroxide.; it burns skin.) Add this slowly to the test tube. This reaction may give off heat. (In other words, it may be exothermic.)
4. Tap the tube firmly to mix or use a stirring rod.
5. Add about 1/3 of a pipet of distilled water. Tap or stir to mix. The solid material is copper(II) hydroxide, Cu(OH)₂(s).
6. Place your test tube in a hot water bath. Remove the test tube when you see no further change occurring. This product is copper(II) oxide, CuO(s). Run cold water over the outside of the test tube to cool it.
7. Allow the material to settle or use a centrifuge to spin the solution. Decant or use the pipet labeled "waste" to remove the clear liquid (supernatant liquid) above the copper(II) oxide. Do not remove any of the solid.
8. Discard the liquid and wash precipitate by adding about half a pipet of distilled water. Tap or stir to mix.
9. Allow the solid to settle. Then remove and discard the liquid, or centrifuge and decant.
10. To the precipitate, add hydrochloric acid, HCl, from the final thin stem pipet. (Caution: Avoid contact of both skin and clothing with hydrochloric acid.) Tap or stir to mix. This new product is copper(II) chloride solution, CuCl₂(aq).
11. Add the precut piece of aluminum wire to the test tube. (The wire should be bent like a fish hook to hook over the top of the test tube.) Place the test tube in a cold water bath. This reaction is very fast! (Instead, your teacher may tell you to run cold water over the outside of the test tube. Be sure to use a test tube holder to hold the test tube since the reaction gives off considerable heat.) The products forming in this step are hydrogen gas, H₂(g), aluminum chloride solution, AlCl₃(aq), and metallic copper, Cu(s).
12. When the reaction is finished, remove and discard the liquid.
13. Wash the solid with half a pipet of distilled water. Discard the liquid.
14. Wash the solid again with half a pipet of fresh distilled water, and filter.
15. Use tweezers to remove any leftover pieces of aluminum wire. Observe the final product.
16. Take the filter paper with your product to the fume hood. Your teacher will test a small sample of your product with concentrated nitric acid.
17. Clean pipets, test tube, and funnel.
18. Wash hands thoroughly before leaving the laboratory.

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Use your observations to complete a drawing similar to the one above. Alongside each arrow, write in the chemicals used. Also near each arrow, write in key words to convey what you saw.
1. As chemical changes occurred, what observations did you note? List the observations that alerted you that some type of chemical change was occurring.
2. Did each reaction show all of the kinds of changes you listed above? Use your observations to justify your answer.
3. What happened to the hydrogen gas generated in the last reaction? Add hydrogen gas to the cycle.
4. What happened to the aluminum chloride? Show this on the cycle.
5. What did you enjoy most about this activity?

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1. Reread the title of this laboratory activity. Why is this series of reactions often called the "copper cycle"?
2. How did the last part of the procedure (Step 16) complete the cycle?
3. Recycling is one important way to conserve precious natural resources.
   1. What common metals have you observed being collected for recycling?
   2. Think about all types of materials that you know are recycled. Compose a list of these materials.
   3. What types of materials might prove too difficult or costly to recycle?

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Teachers Guide

Preparing for the Laboratory Activity

• Major Chemical Concept
• Level
• Expected Student Background
• Time
• Safety
• Materials
• Advance Preparation

Conducting the Laboratory Activity

• Pre-Lab Discussion
• Teacher/Student Interaction
• Anticipated Student Results
• Answers Data Analysis
• Answers to Imply, Apply
• Post-Lab Discussion

Assessing the Laboratory Learning

• Paper/Pencil Items
• Teacher Demonstrations
• Possible Answers
• Grid Assessment of Laboratory Techniques
• Answers

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Preparing for the Laboratory Activity

Major Chemical Concept

Evidence of chemical change can be found through careful observation of a series of reactions involving copper and related copper compounds.

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Appropriate for all levels.

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This activity is performed on a microscale basis, using thin-stem pipets. As written, it can be used by students after the first few weeks of school. With the format used in the student procedure, the activity is not intended to be quantitative. Students should be able to:

• Fold a piece of filter paper
• Separate a solid from a solution by filtration
• Wash a precipitate
• Use a centrifuge (if one is used in the activity)

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Day 1: 50 min.

Day 2: 20 min.

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• Protective glasses and aprons must be worn at all times.
• Equipment such as pre-filled, thin-stem pipets and small volumes greatly reduces safety hazards.
• Remind students that thin-stem pipets are to be used only for experimenting--no squirting!
• Caution students that some of the reactions are quite exothermic.
• Concentrated nitric acid use and the generation of nitrogen dioxide gas are done by the teacher in the fume hood.
• Waste solutions can be poured into the THINK Tank. (see p. 8)

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Non-Consumables (per lab team)

• 1 Beaker, 100 mL - 250 mL
• 1 Centrifuge tube*
• 1 Funnel
• 1 Stirring rod
• 5 Thin-stem pipets
• 1 Tweezers

Non-Consumables (for class)

• Centrifuge (not essential)
• Hot plates or Bunsen burners
• (Students enjoy using a "mixing block" if your lab has one.)

Consumables (per lab team)

• 1 Aluminum wire, pre-cut -- length depends on gauge
• 1 Filter paper circle
• 1 Thin-stem pipet with 1 mL of 1 M Cu(NO₃)₂
• 1 Thin-stem pipet with 2 mL of 2 M NaOH
• 1 Thin-stem pipet with 3 mL of 2 M HCl

A check list of materials per lab team follows:

• 1 Aluminum wire, pre-cut
• 1 Beaker, 100 - 250 mL
• 1 Funnel with piece of filter paper
• 1 Test tube (may be centrifuge tube)
• 2 Thin stem pipets, empty and labeled
• 3 Thin-stem pipets pre-filled and labeled for NaOH, HCl and Cu(NO₃)₂
• 1 Tweezers for removal of excess aluminum wire

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1. Prepare the following (per class of 24 students):
   • About 10 mL of 15 M HNO₃; about 1 g copper turnings
   • 50 mL of 2 M NaOH (Dissolve 4 g NaOH in 50 mL distilled H₂O solution.)
   • 100 mL of 2 M HCl (Carefully add 18 mL of 12 M HCl to 82 mL distilled H₂O.)
   • 30 mL of 1 M Cu(NO₃)₂ (Dissolve 9 g of Cu(NO₃)₂ . 6 H₂O in 30 mL distilled H₂O.)
   • 1 meter of 16 gauge aluminum wire; check the local hardware store. (Aluminum foil is not satisfactory for this procedure since it reacts too rapidly.)
2. Label the thin-stem pipets. Try a letter code on large, colored, stick-on dots. Use clear plastic tape over the dots on the pipets. Save these coded pipets for next time this activity is done. Fill each NaOH pipet with 2 mL of reagent. Fill the HCl pipet with 3 mL of reagent. The initial copper(II) nitrate solution pipet needs 1 mL of reagent.
3. The thin-stem pipets can be stored in a flat position when filled. The empty pipets for distilled water and for removing the "waste" supernatant liquid should also be labeled.

You may need to practice dissolving the Cu turnings in 15 M HNO₃ in the fume hood.

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Conducting the Laboratory Activity

Pre-Lab Discussion

Students should be given an empty pipet and a beaker of water for a brief practice session. Tell them that the pipet is most efficient when held in a vertical position. Remind students to insert a blank, water-filled test tube for balance in the centrifuge as needed.

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During the activity, as you circulate, remind students to record their observations at each step. Expect and encourage very positive comments on the color changes and the wide variety of reaction behaviors. Students gain confidence in the laboratory very early when they are allowed to work with "real" chemicals. The small scale of this activity gives students a chance to develop a responsible, comfortable attitude even toward caustic and corrosive reagents. Insure that protective glasses and aprons are worn at all times.

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1. Red-brown gas; blue solution; heat given off (Step 1)
   
2. Aqua blue solid forms (may contain black flecks) (Steps 2-5)
   
3. Black precipitate (Step 6)
   
4. Clear, mint green solution forms (Steps 7-10)
   
5. Turns grey, "foams", very hot, pink-tan solid (Step 11)
   
6. Product looks like wet compact powder, no luster (Steps 12-15)
   
7. Red-brown gas; blue solution; heat given off (Step 16)

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1. See diagram above
2. Observations
   1. Change in color
   2. Heat given off
   3. Formation of a gas
   4. Formation of a solid
3. Each step did not show all four types of changes. Examples should be given.
4. It escaped into the room.
5. It was decanted off and discarded.
6. Answers will vary.

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1. Copper is taken through a series of chemical changes. At the end, it is recovered as solid copper. The cycle is completed.
2. This step helped provide evidence that the pinkish solid was copper even though it did not look like the copper seen in everyday situations.
3. 1. Other materials that are recycled are aluminum from cans and lead from car batteries. Old car bodies are crushed and recycled as well as copper from old motors. Mercury-containing products and silver from photography developing solutions are also recycled due to the costly nature of these metals.
   2. Paper, glass containers, waste oil from cars, etc.
   3. Some types of plastic containers can be very difficult to recycle. The major problem in recycling many products is the mixed nature of materials. Gum wrappers contain paper and metal foil; flash cubes contain metal and plastic. Manufacturing methods must continue to promote products that can be recycled.

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Allow time during the second period for students to work on the laboratory questions. Exchange of ideas among students at this time is very valuable. Be certain students understand the directions for drawing their results onto the "cycle" in Step 1 of Data Analysis. You may choose to discuss the terms "reactants" and "products" as well as the commonly used symbols "(aq)", "(g)", etc.

For example,

1. Students could be given word equations to complete:
   1. copper + _________ --> copper(II) nitrate + water +
   2. copper(II) nitrate + sodium hydroxide --> + sodium nitrate
2. Students might then be asked to translate those word equations into correctly-balanced chemical equations:
   1. Cu + 4 HNO₃ --> Cu(NO₃)₂ + 2 H₂O + 2 NO₂
   2. Cu(NO₃)₂ + 2 NaOH --> Cu(OH)₂ + 2 NaNO₃
3. Students might be asked to classify reactions, i.e., single replacement, redox, double replacement, etc.
4. This entire activity can be redone later in the year on a macroscale where quantitative results are required.
5. If students have studied the activity series of metals, they can be asked why aluminum is the best metal to displace copper from solution in this activity. Ask them to suggest two other metals that might work (Zn, Mg), two metals that would react too violently (Na, K), and two metals that would not react to displace copper (Au, Ag).

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Assessing the Laboratory Learning

Paper/Pencil Items

Students may use lab notes.

1. Recall what the final copper product on your filter paper looked like as well as the original copper sample used by your teacher. How did the two samples differ in appearance? Try to explain this.
2. Will the chemical reactions of these two copper samples be different? Use your observations to justify your answer.
   • If students have done the extensions of writing and balancing the reaction equations, Questions c and d could be used.
3. In both steps where nitric acid, HNO₃, reacted with copper, the procedure was done in the fume hood. What dangerous product formed that required this safety measure?
4. The aluminum hook was involved in two reactions in the final step of the copper cycle.
   1. Show these reactions as balanced, chemical equations.
   2. Predict what would happen if too little aluminum were available in this displacement step.

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This assessment is done as a series of teacher demonstrations. Students should not use notes for this activity. (Caution them not to look directly at the magnesium ribbon in the flame. Damage to the eyes can result.) You might tell them, "I will demonstrate several reactions. Watch carefully and record what you observe as evidence of chemical change." In doing these demonstrations, use enough materials to make the reactions clearly visible to all students, but avoid excessive amounts. These waste materials can go into the THINK Tank. (see p. 8)

Perform the following demonstrations without comment. Allow enough time for observation and writing. The students need not be told what chemicals you are using. Their task is only to observe and process their observations.

1. Cu wire in silver nitrate solution: This can be done in a Petri dish on the overhead projector. Copper turnings in a 0.1 M solution of silver nitrate work well. The blue color of the Cu²⁺ ion will take about 15 minutes to show up well.
2. Magnesium ribbon in a lighted burner: Use crucible tongs to hold small strips of Mg ribbon in a burner flame. Remind students not to view this directly, but to look off to the side.
3. Addition of Ag⁺ ions to Cl^- ions: The same 0.1 M solutions of silver nitrate used in Part (a) above can be used with any dilute chloride solution. Tap water will often work here as a source of chloride ions.
4. Zinc in hydrochloric acid: Small pieces of mossy zinc react well in 1 M HCl solution.
5. Ammonia added to Cu²⁺ solution: Use some of the original 1 M Cu(NO₃)₂ solution from the student procedure. Add 1 M NH₃ solution in a dropwise manner. First a light blue precipitate will appear. Addition of more ammonia solution will cause a deep, clear blue color to appear.
6. Vinegar plus sodium bicarbonate; CO₂ (g) generated.
7. Cd²⁺ solution added to S^2- solution: This reaction is identical to the reaction pictured on the cover of a popular high school chemistry text. You may prefer not to use this heavy metal. If you use this reaction, you will need only a few milliliters of 0.1 M Cd(NO₃)₂ and a few milliliters of 0.1 M solution of either (NH₄)₂S or Na₂S.
8. Solid Cu in hydrochloric acid: Copper will not displace hydrogen from hydrochloric acid; the reaction should be left until the end of the assessment period. Students may want to recheck this before you collect their papers.
9. Addition of OH^- to Cr₂O₇^2- solution: Dropwise addition of 1 M NaOH solution to 1 M K₂Cr₂O₇ will cause a definite change in color. If you prefer not to use dichromate solutions, addition of the same NaOH solution to distilled water containing the indicator phenolphthalein will produce a change from colorless to bright pink.

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1. Forms pale blue solution / precipitate of silver
2. Energy given off as light
3. A white precipitate forms
4. Gas is given off
5. Aqua blue precipitate / cobalt blue solution
6. Gas is given off
7. Bright yellow precipitate forms
8. No reaction
9. Orange color turns yellow

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1. Match the numbers from the grid:
   1. centrifuge
   2. filter a precipitate
   3. supernatant liquid
   4. water bath
   5. thin-stem pipet
   6. fume hood
   7. exothermic process
   8. precipitate
   9. wash a precipitate
2. Which frames of the grid show techniques of separation?
3. Which frames involve a transfer of energy?

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1. A-6, B-7, C-8, D-3, E-10, F-1, G-5, H-9, I-4.
2. 6, 7
3. 3, 5

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