Trends in the Mass of a Penny

Abstract:
Before the year 1982, pennies were made up of a majority of Cu. However, after that year, the composition of pennies changed to a majority of Zn. Our graph of the masses of ten pennies and the year they were minted in showed a trend in the masses of pennies before the year 1982 and after 1982. In order to find out what caused this change, we used the density of the pennies. Density is an intensive property of a system which means it is “a physical property of the system that does not depend on the system size or the amount of material in the system.” So we took the laboratory further and found the masses and volumes of groups of pennies separated by whether they were minted before or after 1982. By graphing the mass vs. volume of these samples, and finding the densities of each sample, we found that it was a change in penny composition which supports the fact that after 1982, pennies were made with a zinc core and copper coat instead of mostly copper. Also, we proved the changed in composition by putting a nicked old penny and a nicked new penny both into a solution of HCl (hydrochloric acid) and the new penny was the only one to react.
Introduction:
Pennies minted before 1982 were made up of a mixture of 95% copper and around 5% zinc. However, after awhile, making pennies with such a high content of copper became too expensive. Instead, pennies after the year 1982 were minted with a zinc core and a copper coating, around 97.5% zinc and 2.5% copper. The differences in the pennies compositions changed many of their characteristics. Changing the composition of an object and keeping the volumes constant means some properties of the object such as the mass, density and some chemical properties of the object must have changed. So the penny densities were changed and their reaction to HCl changed as well. In this lab, we analyzed penny masses to determine trends between the masses, the years there were minted in, and the causes of these trends. We calculated the densities of the older and newer pennies by graphing our obtained values on a mass (g) vs. volume (mL) graph, giving us the density. The results we obtained explained why the masses before 1982 followed a certain trend in their masses and why after 1982 they followed a different trend. These results supported the historical fact that the composition of pennies was changed that year.
Experimental:
For the first part of the laboratory, we used a 50-mL graduated cylinder, a 50-mL buret and some tap water to find the averaged calibrated volume (V) of the graduated cylinder. First, we filled the buret with water and recorded the initial volume reading to the nearest hundredth of a mL. Then, we filled the graduated cylinder to the 45.0 mL mark without letting the buret volume fall lower than the 50 mL mark and took the final buret volume reading (again, to the nearest hundredth). We repeated the procedure two more times, calculated the calibrated volume for each of the three trials by subtracting the initial V from the final V, and then calculated the averaged calibrated volume of the 3 trials, recording it all in a table. Then, for the second part of the procedure, we used 10 pennies (a mixture of old and new), weighed each one and recorded each mass, along with the year they were minted, in a table. We first found the mass of each of the samples and recorded the masses to the nearest thousandth of a gram. Then we filled the buret with water, recorded the initial buret reading, added a bit of water (around 5 – 10 mL) to a dry graduated cylinder and slid the group of the 4 old pennies into the cylinder, tapping at the cylinder to make sure there were no air bubbles in between the pennies. Then we finished filling the graduated cylinder to the 45 mL mark and took the final buret reading, recording it to the nearest hundredth of a mL We redid this final and initial reading for both of the other groups of