Littlest chemis-tree

Introduction

A meaningful science-based activity on the last day of school before winter holidays is important. At the same time, teachers must recognize that many students will not be present on the last day, or if they are, they are not interested in learning course material. This activity incorporates fun and learning. It can help encourage students to attend class on a day when many teachers resort to showing videos.

Add a drop of clear solution to a microscope slide containing a tiny piece of copper and create a beautiful, silver-branched Christmas tree that grows before your very eyes. This activity was adapted from a ChemFax (The littlest Christmas tree) from Flinn Scientific, Inc. It is listed as Publication No. 10520. Flinn acknowledges Bob Becker, Kirkwood High School, Missouri for the instructions to this demonstration. If you decide to do this activity, this ChemFax has useful information and diagrams.

One addition we made to the procedure is making the slides into tree ornaments. These ornaments were used for our large class tree. The slide was preserved by taping around the edges of the coverslip. If the slides were bent or disturbed, the silver would move and the tree effect would be lost. These little trees did not last long.

Concepts

Proper microscope use, oxidation-reduction, crystallization

Materials

• Ribbon
• Hot glue gun
• Copper strip, Cu, 2 mm x 0.5 mm
• Silver nitrate solution, AgNO₃, 0.3 M, 1 drop
• Microscope
• Plastic microscope slide cut in half
• Plastic coverslip
• Scissors
• Single hole punch
• Tape

Safety precautions

With all chemistry lab activities, goggles are recommended. The silver nitrate solution is moderately toxic by ingestion and irritating to body tissues. Avoid all body tissue contact. Silver nitrate solution will stain skin and clothing. The tiny copper piece is sharp. Be careful when cutting and handling.

Chemis-tree under a coverslip on a microscope slide

Procedure

1.  Obtain the microscope slide and use a hole punch to make a hole through the top of the slide.
2. Cut a 10 cm length of ribbon. Feed it through the hole and tie the ends together.
3. Use another piece of ribbon to make a bow.
4. Use the hot glue gun to glue the bow over the hole in the slide.
5. Make a small tree by cutting out a triangle, about 2 mm tall and 1.0 mm wide, from the copper strip — see above photo.
6. Remove the protective paper covering from both sides of the slide. Hold it only by the edges to avoid putting fingerprints on the viewing area.
7. Place the copper tree in the center of the slide.
8. Place the slide on the microscope stage and focus in on the copper tree using low power.  
9. Remove the protective paper covering from both sides of the coverslip. Hold it only by the edges.
10. Use a small amount of tape to attach the coverslip to the slide on one side.
11. Add a small amount of tape to the other side of the coverslip, but don’t fasten it down yet.
12. Add a drop of silver nitrate solution and tape the other side of the coverslip down.
13. Observe the tree under the microscope and watch as the tree grows branches! (Front cover of magazine.)
14. After viewing, tape all the sides of the coverslip and hang your little tree on the class tree.

Discussion

The “Christmas tree” reaction is a single replacement, oxidation-reduction reaction, in which copper metal is oxidized to copper(II) ions and silver ions are reduced to silver metal. This silver metal forms dendritic silver crystals along the edges of the copper that resemble pine tree needles and branches (from Flinn ChemFax).

Cu(s)  +   AgNO₃(aq) →   Cu(NO₃)₂(aq)  +   Ag(s)

Extra challenge: Balance the above equation