Penny Density Lab Concepts

So, you did the lab, but are not quite sure what to make of this.  Here goes, read this all, and hopefully it contains all the answers you seek (grasshopper).  Anyone get that reference?

The point to the lab was two fold, first to measure the old pennies and measure their mass and volume.  After plotting the data points, we draw a BEST FIT LINE to “average” your measurements into one answer.  The slope of this line on our graph (mass as a function of volume) is the averaged measured density of the old pennies, which you know are made up of only copper.  You will compare your slope/measured density to the density of copper in a percent error formula.  Small percent errors are good.

If your best fit line touches two of your data points (or more) good.  Use any two points ON THE LINE to calculate slope.  If not, you still must CHOOSE TWO POINTS THAT ARE ON THE LINE to do slope.  Points not on the line won’t work.  You can’t pick a point above and match one below either, that will not work.

Then, after measuring the new pennies the same way, and making the same sort of graph with its own best fit line, the slope of this line is the average measured density of the new pennies.  It should be NOTICEABLY less than the copper pennies, as the new pennies are mostly zinc.

Then choose six metals on the periodic table that have density CLOSE TO THE SLOPE OF YOUR NEW PENNIES, not the old ones.  Hopefully zinc is on this short list. Then do another percent error.

Here are some general pointers:

  1.  Only use 8½X11 paper, and your work must be on white paper.  Covers can be colorful, but work is work.
  2. Say what you mean, and mean what you say.  If you tell a lie, I’ll catch it.  Don’t guess, think or call me on the phone for help.  Do not say “I used the density formula to find out it was zinc (I told you it was zinc, you DID NOT use the density formula except for a rando-math problem #8).  You also didn’t use percent error to find out the unknown metal, you used your measured density to see how close you came to the metal I TOLD YOU was inside the new pennies.  Don’t say “I used my measured mass and volume to calculate the volume”, that didn’t happen.
  3. Never say “it” or “they” or vague terms like that.  This is science, not the commons at lunchtime.  Be specific.  Imagine this is not being graded by your teacher, rather you are trying to PROVE science to a nonbeliever.  Use facts, be clear, use your data, and don’t be vague. 
  4. Use numbers in your conclusion!  Tell what you got, tell what your percent error was.  Use your numbers to back up what else you conclude.
  5. SPECIAL SENTENCE:  Since density is a constant, I can make careful measurements and use density to discover what an unknown is, or is not, in lab.
  6. Significant figures are aptly named, they always count.  You must always think about them, always and forever.
  7. Write out formulas, especially when I tell you to.  Don’t discount me, until you get your Nobel Prize, but not before.
  8. Make sure your units cancel out, or else your equal sign is a lie.  Respect the equal sign, it is the most profound symbol in math.
  9. Spelling errors are pointed out for only one reason, to show you I am reading.  Spell correctly best you can, but know that I am into this, I read, and try to believe what you say (re-read #2 above).
  10. All graphs must have titles that are meaningful.  Old Pennies is not a title of a graph, but it might be the title to a children’s book about coin collecting, or some old horse.  They also get axis labels, with units.
  11. Units is my middle name, I take them seriously and so should you.
  12. Scales on graphs must be consistent and you are not allowed to miscount, that means you are not paying attention or do not understand, which means you could not possibly get full credit.
  13. Skip this one, okay?
  14. When I ask you to fix things, you should.  When I tell you to fix your lab WITH ME, consider for a moment if I would lie to you, no.  You need help and I know the help you need and I get paid to help you, and you are charged to learn the chem.  If you are not getting it, I know you are not getting  it, let me do my magic and help you understand.  This is not my first rodeo, I see things about your thinking that even you don’t.  See you soon.
  15. Read the rubric every time.  Every lab gets a cover.  The cover for this lab was worth 2 points out of 25.  Why would you NOT DO THIS?  Maybe because you didn’t read the rubric.  That’s an educational word that really means:  HOW TO SCORE SOME POINTS.
  16. No blanks, they are always wrong.  Always.
  17. If your handwriting is terrible, well, tough on me.  Consider typing your conclusions.  Do your questions in order, and PLEASE, please, leave more space between the questions so I can fit in some comments back to you.
  18. You must use points on the graph to do slope, not your data points that are not on the best fit line.
  19. Always end with “I love chem, the end.” please.
  20. Check my math, I found a lot of booboos, that I fixed using your data.  You did better than you thought, but you still need to do the math correctly.  You can never say you did math wrong as part of your percent error, you are not allowed to do math wrong.
  21. Liquid ≠ AQUEOUS.  Liquids are pure substances that are above their melting points and below their boiling points.  Aqueous are substances that are dissolved into water, which makes a mixture, not a “substance”.  This is not from lab, but if you read this far, this is good to remember as well.

I appreciate that you are working hard, and that you are trying.  You don’t lose a lot of points for dopey mistakes (yet) but you must work to better yourselves too.  Fix what you can, grow from these early, easier lab reports, to better your learning, and your life.

Thank you for being my students.