Name__________________________ Enriched Science 7 – Laboratory Investigation Date: __________________________ Gold Pennies Background: There’s just something strange about a gold penny. First of all, gold pennies don’t exist, unless you’re a 21’st century chemist who knows something about the science of alloys. Alloys are solid solutions of metals or nonmetals dissolved in metals. Chemists, modern day alchemists, cleverly disguised as chemistry teachers, often share the secret of making gold pennies that become keepsake pennies that can never be put back into circulation. Even though it’s a “gold” penny, the US Treasury has a little problem with people trying to buy something with gold pennies. Gold is roughly worth $53.00 per gram and copper is worth about a penny per gram. In 1983, the US Treasury started making pennies with a zinc core and with a copper coating. As it turns out they weigh less and cost less to make than their pre-­‐1983 counterparts, which were made with an alloy of 95% copper (copper is too expensive now) and 5% zinc. In this activity, soaking the penny in the zinc solution actually coats the surface of the penny with zinc atoms. When the zinc covered penny is heated, the copper atoms of the penny and the zinc atoms coating the penny mix and turn gold in color. They are physically combined. This mixing of metals is an alloy called “yellow brass.” Artist often work with alloys like bronze, steel or brass because of their durability and color. Bronze is a mixture of copper and tin. Brass, made in this activity, is a mixture of zinc and copper. Steel is made of iron and carbon. Alloys are used to make coin coins, jewelry, sculptures, and other items. Materials Reactants; 20 ml of NaOH, 1 g Zinc Dust, Post 1983 Penny Materials; safety goggles, Bunsen Burner, cloth/rag, evaporating dish, tongs, balance, chemical spatula, ring stand, graduated cylinder, weighing paper, forceps, and 200 ml beaker of water Safety Note: NaOH should be handled with great care. It is corrosive and can burn skin. Zinc dust should not be inhaled. Safety goggles should be worn the entire time. Procedures: 1. Measure 1g of zinc dust on a balance in a beaker. 2. Pour 20 ml of NaOH solution in the graduated cylinder into the beaker with zinc in it. 3. Carefully place your penny into the beaker with a tong. 4. Heat for about 5 minutes on the hot plate. Do not boil. Heat until the penny is coated and becomes silver in appearance. 5. Then remove the penny from the beaker with tongs or forceps, and drop it into a beaker of water. Remove the penny from the water when cool, and wipe the penny clean with a cloth to remove any excess zinc. 6. Using tongs or forceps, hold the penny in the flame of a Bunsen burner and gently heat. The penny should turn “gold” (brass). (Do not overheat the penny.) 7. Dip the penny in the beaker of water until it is cool to the touch. 1
Conclusion 1. If alloys are solid solutions of metals or nonmetals dissolved in metals, which of the metals in this activity is the solvent and which is the solute? Check page 93 in your text book to help you answer this question. Answer: a) solute = ______________________ b) solvent= ______________________ 2. As stated in the background information, the “gold” pennies created in lab are only keepsakes, and can no longer be used as “money.” Why do you think the US Treasury would have a problem with people using these “gold” pennies as real gold to buy something? ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ 3. Yellow gold is an alloy of gold and silver (also some copper and zinc). Go on line and research yellow gold to determine which metal is the solvent and which is the solute and report your findings on the lines below, along with the resource that provided your answer. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ 2