Density Lab

The density of a pure substance does not depend upon the amount of that substance. Like temperature or melting point, density is an intensive property. That is, a ton of lead has the same density as a milligram of lead (11.63g/cm3). Common units for density are grams per cubic centimeter for solids, grams per milliliter for liquids, and grams per liter for gases. In this lab we will work with solids.

Purpose:
To develop the formula for density from experimental data by graphical interpretation, and to make comparisons in the densities of two different metals.

Materials:

various sized graduated cylinder (10-100 ml)
6-8 metallic objects
centigram balances

Procedure:

1) Pick up a container holding 8-10 metallic objects. These objects are not marked. It will be the duty of one person to be sure to keep track of which objects have massed and keep the process orderly.

2) First determine the masses of each object by laying it on the balance pan and determining its mass. Be sure that the metallic object is dry.

3) After each object's mass has been determined, carefully lower the metallic object in to a graduated cylinder (choose a cylinder slightly larger than the piece, a 25 mL may work for all pieces) containing an accurately measured amount of water. I suggest you have 5.0 ml of water already in the graduated cylinder. Be sure that the metallic object is completely submerged in the water before taking a new reading off the graduated cylinder. This method of determining the volume of an irregularly shaped object is called volume displacement.

At the end of the writing is an example data table for you to use in recording your experimental data.

Make a graph of mass versus volume. Record the mass along the vertical axis and the volume in ml along the horizontal axis. Plot all data points on the graph. Give the graph a title; follow accepted graphing procedures.

Findings:

  1. Is the origin (0,0) a valid point? Explain.
  2. Draw a line or lines that best "fits" the data. The line or lines should be a straight line.
  3. How many lines were plotted on the graph? What importance can be attached to this observation to help make sense of the experiment?
  4. The formula for a straight line is y=mx+b. What is the value (number reading) of b on your graph? Since b = ____, the y = ____.
  5. Substitute into the amended equation mass for y, and volume for x. Then solve for m, the slope of the line.
  6. Identify a physical property equal to the solution for m in question 4?
  7. On your graph draw a perpendicular line form the 4-ml mark on the x-axis until it intersects the plotted lines. From the point of intersections, draw a line that is perpendicular to the y-axis. Using axis readings, determine the density represented by each slope?
  8. Use the data table to determine the densities of each metal sample. How do these values compare with the density values determined by reading the graph.
  9. A liquid occupies 12.6 ml has a mass of 11.2 grams. Find the density of the liquid.
  10. Write a summary that explains what you learned from doing this experiment.



Prepared for SMART Center Workshop, July, 1996.
Revised 7/5/96.
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