1. How does the Mass of a Penny Change with Age?
Analyzing variation in data

2. Mint Not very good (2000 years old)
Will the masses stay the same, increase, decrease or show random fluctuations?

3. Data Analysis: 5 brand new pennies in mint condition measured on the same balance
How can we explain the variation in mass?

4. Concept of Uncertainty
Every measuring device has a natural range of possible error (even very expensive electronic balances):
Experimental Uncertainty for a high quality instrument is small but is never zero.
Uncertainty of our balances = g

5. Uncertainty in a Measurement
Uncertainty – Range of possible error
Example: g g
(most probable value with a range of possible variation)
Means the true value lies within range g g g
Most probable value

6. Sources of Experimental Uncertainty in a Digital Electronic Balance
1) Readability and repeatability of instrument
Example Causes in electronic scale – variation in value of electric current flowing through instrument, vibrations on table, rounding of last digit according electronic signal input
2) Changes in environmental Conditions
Examples: Changes in Temperature, Air pressure, air currents, humidity, exposure to direct sunlight
3) Consistency of Operator Technique
Is the penny placed in the EXACT same position on the scale in the same way every time?

7. Consistent with hypothesis that some pennies become scratched and thus lose mass over time
Outside of uncertainty range

8. Consistent with hypothesis that mass of pennies does not change
Variation is within experimental uncertainty; No
Variation is within experimental uncertainty: We cannot say for sure that 2.49 and 2.51 values are truly different.

9. Consistent with hypothesis that some pennies gain mass over time (perhaps due to dirt or surface oxidation)
Outside of uncertainty range

10. Consistent with hypothesis that some pennies gain mass and some loss mass depending how they have been handled

11. Best Analysis of Data is to collect larger data sets in order to see patterns more clearly:
Clear variation both above and below most probable values is present: Some pennies obviously gain mass and some lose mass over time.
How can we explain large gap between 2.60 and 2.99 g in data set?

12. One way to simplify a large data set is to chose a representative value for the set, such as the average mass of a large number of pennies for the entire year. When plot the data in this way, average mass vs. Mint Year we get this graph. Propose a hypothesis to explain this result. Propose an experiment to test your result.

13. Interpretation:
The change in average mass up to 1982 and after 1982 is far too large (≈ 0.60 g) to be attributed to experimental uncertainty ( g) or scratches, oxidation or dirt and is too consist (all pre-82 ≈ 3.1 g, all post 82 ≈ 2.5 g) The chemical composition of the pennies must have changed.

14. Experiment to test hypothesis:
1) Compare physical properties such as melting point of pre-82 and post -82 pennies.
Melting point Cu =1984 oF (1085 oC)
Melting point Zn = 787oF (420 oC)
Recall Bunsen burner flame temp. ≈ 600 oC
2) Compare chemical reactivity of pre-82 and post 82 pennies.
Zn reacts with hydrochloric acid, Cu does not.
Video link to post -82 HCl

15. In 1982, the US mint changed over from pure copper pennies to zinc filled penny with a copper coating because the cost copper made the metal in a pure copper penny worth more than 1¢. A cutaway view is given below: