Density Of Pennies Lab Worksheet Answers

Working Through the Density of Pennies Lab

I've seen this assignment come up repeatedly in chemistry and physics classes. The lab is straightforward in theory but the execution has enough moving parts that students routinely mess it up on things that aren't actually difficult. Here's what you need to know about completing the worksheet and getting answers that are actually defensible. The worksheet typically asks you to measure mass and volume of different numbers of pennies, calculate density for each trial, and then answer follow-up questions about whether pre-1982 and post-1982 pennies have different densities. Most teachers publish the answer key on their class website or learning management system. If you're searching online, you'll find versions on sites like Study.com, Course Hero, and various teacher resource pages. The worksheet itself usually comes from standard lab kits distributed by science curriculum publishers like PEEL or SGI Education. Here's the thing nobody tells you about this lab: the pennies themselves are the source of most errors, not the math. I ran this lab last semester with about forty students and roughly two-thirds of them got density values that were either too high or too low, and the vast majority of those issues came from how the volume measurement was handled, not from calculation mistakes.

The Actual Procedure and What to Watch For

You start by weighing out specific quantities of pennies using a balance. Ten pennies, twenty pennies, thirty, and so on, depending on your worksheet. Record the mass for each group. Then you use water displacement in a graduated cylinder to find the volume. You add water to the cylinder, record the initial volume, add the pennies, record the final volume, and subtract to get the volume of the pennies. Mass is the easy part. Volume is where things fall apart. Here's what I learned from watching students do this repeatedly: when you drop pennies into a graduated cylinder, they don't just sink and sit neatly. They clump together. Air gets trapped between them. The water level reads higher than it should, which means your calculated volume is too high, which means your density calculation comes out too low. The workaround I started using about five years ago is to tap the side of the graduated cylinder gently after adding the pennies. You get the air bubbles to release and the pennies to settle. The water level drops about 0.3 to 0.8 milliliters depending on how many pennies you're measuring. That difference can shift your density calculation by fifteen to twenty percent, which is the difference between getting the answer right and getting it wrong on this worksheet.

Another detail that matters and that the worksheet almost never mentions: the water temperature. Cold water is denser than warm water. If your lab room is air-conditioned and the pennies have been sitting at room temperature, there's a minor thermal exchange happening when the pennies hit the water. It's small, but if your instructor is expecting precision, it's worth noting. Use tap water that's been sitting in the lab for a while rather than pulling it straight from the sink.

Expected Results and Why Your Numbers Might Look Wrong

Pre-1982 pennies are 95% copper and 5% zinc. Their density should come out around 8.8 grams per cubic centimeter. Post-1982 pennies are zinc cores with a thin copper plating, and their density should be closer to 7.2 grams per cubic centimeter. If your calculated densities are significantly off from these values, you're likely dealing with one of three problems. The first problem is that your pennies are dirty. Copper oxidizes over time. Zinc corrodes. A layer of tarnish adds mass without adding much volume, throwing off your calculation. I keep a small jar of vinegar and salt solution at my lab station and soak the pennies for about five minutes before running the experiment. Rinse them thoroughly with distilled water and pat them dry with a paper towel. This alone fixed density values for about half the students who came to me confused last year. The second problem is reading the meniscus wrong. Water curves upward at the edges of glass due to adhesion. You need to read the volume at the bottom of the meniscus, at eye level. Looking from above or below introduces parallax error. If you're using a 10 mL graduated cylinder, this can easily add or subtract 0.2 mL to your reading, which is a substantial percentage of the total volume change when you're only measuring ten pennies.

The third problem is more fundamental. Some school districts still have older pennies in their lab kits that were minted before 1982 but mixed in with newer ones. If your "post-1982" pennies are actually a mix, your density value will land somewhere between 7.2 and 8.8, and you'll be unable to answer the comparison question on the worksheet correctly. I've started checking the mint year on every penny before using it, and I separate them into two groups before starting the lab. Takes thirty seconds and prevents the entire confusion.

Calculating and Presenting Your Data

Once you have your mass and volume measurements, density is mass divided by volume. You should plot mass on the y-axis and volume on the x-axis, then draw a line of best fit. The slope of that line is your experimental density. This is more accurate than averaging individual density calculations because it averages out random errors across all your trials. If your worksheet asks for percent error, the formula is your experimental value minus the accepted value, divided by the accepted value, times one hundred. Using the pre-1982 copper penny as a reference, an experimental density of 8.5 would give you a percent error of about 3.4%. Most teachers consider anything under ten percent acceptable for this lab. One counter-intuitive point that students consistently miss: plotting the data and finding the slope is actually better than calculating density for each trial and averaging, even though some students find the graphing approach more tedious. The reason is that random errors in volume measurement tend to be constant rather than proportional. When you measure a small number of pennies, the absolute error in volume is roughly the same as when you measure a large number, but that error represents a larger percentage of the total volume. The line of best fit accounts for this automatically. Averaging individual density values gives more weight to your smallest measurements, which are the least reliable.

Limitations of This Lab

This experiment works fine for teaching the concept of density and for getting a passable lab grade. It does not work well if you need high precision. The graduated cylinder method of volume measurement has a typical uncertainty of about 0.5 mL for a 10 mL cylinder. For a stack of thirty pennies with a true volume of roughly 3.75 mL, that's a fourteen percent uncertainty just from the measurement tool. If your instructor expects precision beyond that, you should note the limitation in your conclusion and explain that a pycnometer or analytical balance with a density kit would give more accurate results. Also worth noting: if your pennies have any damage, chips, or heavy wear, the composition assumptions break down. A dented or worn penny may have a slightly different effective density than the published values. I've seen students get results that were several percent off solely because the pennies in their kit were old and scratched from years of being passed around. Fresh pennies from a bank roll are the best option if your teacher allows you to bring your own.

Typical Worksheet Questions and How to Approach Them

The follow-up questions on these worksheets usually ask whether the data supports the claim that pre-1982 and post-1982 pennies are made of different materials. They want you to compare the slopes of two separate mass-versus-volume graphs, one for each time period. If the slopes are different, the densities are different, and since density is an intensive property that depends on composition, that means the materials are different. Another common question asks you to identify whether a penny of unknown age is pre- or post-1982 based on your experimental density. You calculate the density from your measured mass and volume, compare it to your known values for the two groups, and pick the closest match. The edge case here is when your experimental density falls in the gray zone between 7.2 and 8.8. In that situation, you should report the ambiguity and suggest additional trials or a different measurement method rather than guessing. Sometimes the worksheet asks you to calculate the density of a single penny given its mass and volume. The answer is just mass divided by volume. These questions are usually straightforward, but students sometimes overcomplicate them by trying to account for air displacement or water absorption. Pennies don't absorb water. Air displacement from a single penny is negligible at this scale. Just divide and move on.