Question

how were we able to demonstrate in lab that pennies minted after 1982 have a different composition than those minted before 1982

Answer 1

Before 1982 pennies were made up of 95% copper and 5% zinc whereas after 1982 pennies were made up of 97.5% zinc and 2.5% copper (copper-plated zinc).

In the lab, by finding mass and volume of both pennies before and after 1982 and plotting mass versus volume for both copper and zinc independently, density for both copper and zinc will be obtained. From which it can be calculated that there is a difference in the composition of pennies before and after 1982.

The amount of copper in pennies can be determined by soaking both pennies in HCl solution. Copper does not react with HCl and Zinc reacts with HCl. When new pennies were soaked in HCl solution, zinc gets dissolved and leaving a copper shell. And finally performing gravimetry analysis of penny prior and post.

Another method is to add a penny to concentrated nitric acid were all copper gets dissolved. Then water and ammonium hydroxide were added to a beaker containing copper dissolved and followed by dilution, its absorbance was measured at 620nm using spectrophotometry to obtain a quantitative measurement of the intensity of the light as a function of wavelength from which concentration of copper can be known.