High School Biology-Chemistry SMILE Meeting
12 October 2004
Notes Prepared by Porter Johnson

Ron Tuinstra [Illiana Christian HS, Chemistry]                   Density and Significant Figures (Handout)
Ron handed out a sheet with the following exercises (we did the first two in today's class):

  1. Find the density of one of the density cubes using a ruler (always metric) and the electronic balance. Remember to measure correctly and use correct significant figures. Compare your answer to the correct answer and find the error.
  2. Determine the thickness of aluminum foil by finding the mass, density of aluminum (from chart), length, and width. Again, use significant figures properly.
  3. Find the density of an unknown irregular solid using a graduated cylinder and electronic balance. Be sure to measure correctly and use correct significant figures. Identify the solid by comparing the density to the chart given. Calculate the percentage of error.
Ron had ten cubes of identical or nearly identical size, made of various materials, and these served as the "experimental materials" for the first exercise. Our density measurements for materials were off (compared to known values for each material) usually by 5-10 %. We checked our balance with standard weights, and the balance was fine. We guessed that probably our dimension measurements may have been the source of the error.

For the second exercise, we use the equations for density D in terms of mass M and volume V of a material  D = M / V. In addition, we used the formula for the volume V of a rectangular sheet in terms of its length L, width W, and thickness T; namely  V = L ´ W ´ T. Once we measure the width W and length L  of a rectangular piece of aluminum foil, we have all the information  needed to calculate its thickness:

T=M / [L ´ W ´ D]
Matt Collier, Chris Etapa, and Ken Schug all calculated the thickness of aluminum foil at T = 0.16 mm = 0.006 inches, which seems quite reasonable (we do not have an independent value for this thickness).  Good stuff, Ron!

Ken Schug  brought a sealed silvery glass vial into which we could not see. Ron rocked it gently and it seemed to have a solid in it, but when the vial was warmed in Ron's hand for a minute or two, we could see that there was a liquid inside. It was Cesium metal inside, which has a melting point just above room temperature.  Caution:  Cesium metal is highly corrosive -- do not come into contact with this substance!

Marva Anyanwu [Wendell Green Elementary School]         Sinking a straw (Handout)
Problem:  Determine the number of BBs necessary to sink a straw to any chosen depth in water.
Materials:  sinking straws, BBs (#9 lead shot), metric ruler, beaker, water, small rubber band, modeling clay (for plugging straw)

  1. Cut the straw to about 12 cm in length, and plug one end with a small amount of modeling clay.
  2. To serve as a mark, wrap the rubber band around the straw about 4 cm from the plugged end.
  3. Determine the number of BBs required to sink to the straw to that mark, and record your answer in a chart for recording data.
  4. Move the rubber band up to 5 cm from the plugged end of the straw, and find the number of BBs required to sink the straw to the 5 cm mark.  Record your answer.
  5. Predict the number of BBs needed to sink the straw to a 6 cm mark. Test your prediction by marking the straw at 6 cm, and find the number of BBs needed to sink it to that mark.
  6. Repeat the previous exercise for a 7 cm mark.
  7. When you are ready, get a "challenge depth" from your teacher.  Predict the number of BBs needed to sink the straw to the challenge depth, and determine the number actually needed.  Did you get it?

We carried out this exercise in class with enthusiasm, obtaining good results.  Thanks for sharing this, Marva!

Notes prepared by Benjamin Stark.