PK-3rd Grade
A metal named Zinc covers the surface of a penny to change its color. This causes the color of the penny to change from copper color to a silver color. Heating the penny causes an alloy to form that causes the color to appear gold. Students at this level understand that there are three states of matter: solids, liquids, and gases. In this experiment, a liquid (sodium hydroxide) and a solid (zinc metal) react with a penny to change the appearance of the penny.
Grade 4 – 6 students
Zinc metal is used to change the color of a penny to silver. If placed in an open flame, the silver penny can be heated to melt the zinc and copper together to create a golden color. Students at this age group understand that all substances are made up of atoms and that atoms combine to form molecules. Zinc and copper are elements which are each a single kind of atom. They understand that during chemical reactions the atoms in the reactants rearrange to form products with different properties. In this case, the heating of a penny in the presence of zinc causes a coating of metal on the surface of the penny that appears silver in color. Heating of the penny in a flame causes the color to appear gold
Middle/High School
Zinc metal plus sodium hydroxide is used to plate zinc metal on the surface of a copper penny. The sodium hydroxide and zinc metal react to allow zinc metal to be formed on the surface of the penny. Further heating of the “silver” penny causes the zinc and copper to react to form bronze which appears gold in color. Students at this level know the structure of the atom and understand that it is composed of protons, neutrons, and electrons. They know that compounds are formed by combining two or more different elements and that compounds have properties that are different from their constituent elements. In the case of this activity, the copper and zinc atoms combine to form an alloy. The properties of alloys depend on the metals mixed together and their relative proportions.
College/Scientific Level
Sodium hydroxide reacts with zinc metal to allow it to plate the surface of a copper penny. The silver color that is visible on the penny is white brass, not silver metal or pure zinc. Zinc is a metal that is easily oxidized allowing it to transfer its electrons to other substances. Copper, in contrast, is not as easily oxidized and, for this reason, it is used as a coinage metal. Up to 1837, pennies were made of pure copper. Since then, pennies have been made as alloys. Between 1864 and 1962, pennies have been 95 percent copper, and five percent tin and zinc. The high cost of copper has caused a change in the composition in 1982 to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). For this reason, this experiment works best on pennies that are dated prior to 1982 which have a high copper content.