A GREEN PENNY??

Knight Foundation Summer Institute

Arthurea Smith, Strawberry Mansion Middle School

Liane D'Alessandro, Haverford College

Introduction:

This is a great experiment to introduce students to chemical reactions. The results which come in less than a week are very exciting to observe. Essentially what happens is that the copper in the penny reacts with the oxygen in the air to form the green colored copper oxide.

To spark the students' curiosity even more try simply giving them the directions for the experiment and tell them to record careful observations over the next week. This way, the students will not know in advance that some change is necessarily going to occur. After they have completed their observations, then introduce the concept of a chemical reaction. A chemical reaction is the combination of two reactants to form an entirely new product. Bonds are broken and new bonds are made. Simply mixing two substances together does not necessarily cause a chemical reaction. For example, combining different kinds of vegetables to make a salad is not a chemical reaction; it is just a mixture of vegetables. The properties of the individual vegetables do not change. Furthermore no bonds are made or broken. Examples of chemical reactions and mixtures are given in the "Assessments" section.

Objectives:

  1. to understand the difference between chemical reactions and mixtures
  2. to understand the difference between a reactant and a product
  3. to explain the effects of acid rain on statues and other objects in our environment

Vocabulary:

chemical

reaction

mixture

reactant

product

Materials:

for each group of students:

Procedure:

  1. Place a penny in each beaker.
  2. Sprinkle 1/2 tsp. of salt over the pennies in two of the beakers.
  3. Pour in vinegar to cover one of the pennies with the salt and one of the pennies without salt.
  4. Label these two beakers with the date and contents (e.g. "2/12/98 -- vinegar and salt" or "2/12/98 -- vinegar").
  5. Pour in water to cover the other two pennies (one with salt and one without). Label each of these beakers with the date and appropriate contents.
  6. Record how each beaker looks (i.e. -- the penny and the liquid).
  7. Continue recording observations of each beaker every day for the next week.

Assessments:

Have the students discuss their observations in their groups (i.e. -- which pennies turned green, which ones turned green first, etc. . . ) F.Y.I. -- they would all turn green eventually; however, vinegar and salt makes the reaction occur even quicker.

Give the students examples of chemical reactions and simple mixtures and see if they can distinguish between the two. Have them discuss their answers in small groups then as a class to see where their lack of understanding (if any) is coming from. For example:

MIXTURES:

  1. cereal (wheat flakes and milk)
  2. lemonade (lemon juice and sugar)
  3. salad dressing (oil, vinegar, and spices)
  4. chocolate milk (chocolate syrup and milk)

CHEMICAL REACTIONS:

1. baking soda and vinegar -- see "Volcano!" experiment

NaHCO3 + CH3COOH NaCH3CO2 + CO2 + H2O

sodium bicarbonate and acetic acid yields sodium acetate, carbon dioxide, and water

2. vinegar and calcium carbonate -- see "The Egg-citing Egg-speriment!" experiment

CaCO3 + 2CH3COOH Ca(CH3COO)2 + H2O + CO2

calcium carbonate and two acetic acids yields calcium acetate, water, and carbon dioxide

3 iron and oxygen (rust) -- see "Must it Rust?" experiment

4Fe+3O2-->2Fe2O3

four irons atoms and three oxygen molecules yields two iron oxide molecules

4. Green pennies -- see this experiment

2Cu+O2-->2CuO

two solid copper atoms and one oxygen molecule yields two copper oxide molecules

 

Extensions:

Try the experiments cited above in the "Assessments" section for other exciting examples of chemical reactions.

Incidentally, the reason that the Statue of Liberty turned colors from its original copper to its present day green is due to a somewhat similar reaction. The statue would naturally have turned that color due to simple exposure to the oxygen in the air. However, the acid in acid rain speeds up the reaction just as the vinegar (acetic acid) caused the reaction with the penny to occur in less than a week.

Philadelphia Science Content Standards:

SCIENCE CONTENT STANDARD # 1: NATURE OF SCIENCE

This experiment satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."

SCIENCE CONTENT STANDARD #2: PHYSICAL SETTING
This experiment satisfies Benchmark 5 for grades K-4 because it gives students the opportunity to "observe how materials change over time". In this case, the students see how copper statues can change over time.

SCIENCE CONTENT STANDARD #5: DESIGNED WORLD

The introduction to the Philadelphia Science Content Standards states that students should understand "the impact of science and technology on society." If connections are made to how the reaction of acid with copper occurs in nature (i.e. the Statue of Liberty), then students will gain an appreciation of how scientific concepts have relevance in society.

Cross-references:

This experiment could be used in a chemistry unit on chemical reactions. It could also be tied in with a lesson on weather (acid rain) or social studies (The Statue of Liberty).