Purpose and Background: Density Gradient Column Lab To create a density gradient column similar to a method used by Forensic Scientists so that the density of various objects can be explored and compared. In the second century BC, the great mathematician and scientist, Archimedes, observed and measured the behavior of various substances when immersed in water. Archimedes' Principle states that:...any body completely or partially submerged in a liquid is acted upon by an upward (buoyant) force which is equal to the weight of the liquid displaced by the body. A block of wood floats because the weight of the block is less than the weight of an equal volume of water. If the block of wood is totally immersed in water, the upward (buoyant) force is greater than the block's weight, and the block will rise up out of the water until it is displacing a volume of water whose weight is exactly equal to the weight of the block. Objects that float are less dense (less compact) than the liquid that they're floating in. A rock sinks in water because the weight of the rock is greater than the weight of an equal volume of water. A rock immersed in water will weigh less (by an amount equal to the buoyant force), but since the buoyant force is less than the weight of the rock, the rock will still sink. Objects that sink are more dense (more compact) than the liquid that they're immersed in. An object with the same density as the liquid surrounding it will remain suspended in the liquid. In Forensics, density is used to compare glass and soil samples. A density gradient column is a quick and easy way to compare multiple samples at one time. In this lab you will create your own density gradient column and test some sample objects using common items from your home. Materials: You will need EACH of these items: Tall, narrow, clear container such as a tall drinking glass Rubbing (Isopropyl) Alcohol Vegetable Oil Water Dish Soap Milk Maple Syrup OR Agave Syrup Honey (Alternatively, you can use Molasses or Corn Syrup) 2 different glass samples (Examples: glass marble, small glass trinket, glass bead, small glass mosaic tile, piece of broken glass). *Use CAUTION when touching a glass fragment with sharp edges! Spoon
Choose 10 of the following: Screw or nail Piece of aluminum foil rolled into a small ball (size of dime) Popcorn kernel Die (dice) Cherry or grape tomato Grape Olive Plastic bead Soda cap Ping pong ball Almond or peanut Small wood chip Raisin Chocolate chip Marshmallow Gummy candy (such as gummy bear) Ice cube Dry pasta (small piece) Crayon piece Procedure: Note: It is VERY important that you pour the liquids in the order as directed below, so follow the procedure very carefully. The final product should look similar to this:
1. Gather materials as indicated above; ensure that you have all of the items listed in the first set of materials and that you choose 10 of the items in the second set of materials! 2. Pour honey into the tall, clear container. Be sure that you don t get honey on the sides of the glass or it may cause issues in your lab as you pour the other liquids. Fill the bottom of the glass until the honey reaches a height of about ½ to 1 inch. 3. Hold the spoon close to, but not touching, the honey, and pour the Maple Syrup (or Agave Syrup if that is what you chose) over the spoon on top of the honey. The spoon keeps the syrup from disrupting the honey layer. The syrup should pool on top of the honey layer. Pour the syrup until it reaches a height of about ½ to 1 inch on top of the honey. Wipe or rinse the spoon clean for the next step. 4. Carefully pour milk slowly over the spoon so that it does not disrupt the syrup layer. Continue until the milk reaches a height of ½ to 1 inch above the syrup. Rinse spoon. 5. Continue the pouring method as directed above for the liquids in this order: Dish Soap, then water, then vegetable oil and finally alcohol. Be sure to pour slowly and carefully over the spoon each time so that you disturb the layers below as little as possible! 6. Once you have finished pouring the liquid layers for the density gradient, let it settle for 2-3 minutes while you complete your density predictions about the objects you chose for this lab in Table 1. Be sure to complete Table 1 before you begin putting objects in the column. At the bottom of the Table 1, you will see Glass Sample 1 and Glass Sample 2 ; you only need to indicate in which layer in the column that you think each glass piece will sink/float or suspend in. 7. Drop each object slowly and carefully into the density gradient column. Try to drop it as close to the surface as you can so that the layers are disturbed as little as possible. Let each object settle into a layer before dropping the next object. Once you have dropped all 10 objects into the column, allow it to settle for 3-5 minutes in case some of the objects are moving slowly through the column. 8. Observe the location of each of the objects and record as indicated in Table 2. Use the box of density values above Table 2 to assist you in completing Table 2. Complete all sections in Analysis and Conclusion in the lab. 9. Once you have completed all of the lab data tables, analysis and results/conclusion sections, you may dispose of the column contents in the garbage. Do not pour it down the drain! Finally, clean up your clear container and lab space.
Data: Table 1 Object Make a prediction and rank each object by density 1-10: 1= Least Dense Object and 10=Most Dense Object Why did you rank the object this way? Predict which layer each object will suspend itself in: (Example: Honey layer) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Glass Sample 1 Glass Sample 2 Density of the Column Liquids: (For your reference) Honey Maple or Agave Syrup Milk Dish Soap Water Vegetable Oil Alcohol 1.42 1.37 1.03 1.02 1.00 0.92 0.79
Table 2 Object Layer it ended up in: Density of that layer: Did Object Float, Suspend or Sink? Approximate density of the object: Is the object more dense, less dense or the same density as water? 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Glass Sample 1 Glass Sample 2 Results: Why is a Density Gradient Column a useful way to compare the density of multiple objects?
Analysis and Conclusion: 1. What is density and how is it mathematically calculated? 2. Look over your predictions in Table 1. Were you surprised by the results of the density column objects? Why or why not? 3. Explain what it means when the following happens: A. An object sinks to the bottom of the column: B. An object is suspended in a liquid in the column: C. An object floats on the top layer of liquid in the column: 4. If you have a piece of glass from a crime scene that is broken into 4 pieces, what happens to the density in each of those 4 pieces? Why?
5. Calculate the density of the following Glass samples: A. Sample 1: Mass= 86.8 g Volume= 28 cm 3 B. Sample 2: Mass= 85 g Volume= 34 cm 3 C. Sample 3: Mass= 49.06 g Volume= 22 cm 3 D. Sample 4: Mass=116.16 g Volume= 48 cm 3 6. Identify the type of glass in each sample above using the following density values: Glass Type Density (g/cm 3 ) Borosilicate 2.23 Tempered 2.42 Common Glass 2.5 Lead Crystal 3.1 Sample 1 Type of glass Sample 2 Sample 3 Sample 4
7. Indicate where each sample would end up in the Density Gradient Column below. The liquid in each layer is labeled along with the density for that layer. Sample 1 Sample 2 Sample 3 Sample 4 8. Why would the above density gradient column not be helpful in all of the sample glass comparisons?
9. What might make it more accurate? 10. Summarize in at least 4 sentences what you learned in this lab: