Summary

In this lab, students will use common laboratory equipment to devise a method to measure the density of several irregular objects. They will then create a formal laboratory report using both their own data and data from the entire class.

Grade Level

High school

Objectives

By the end of this lab, students should be able to

• Determine a method to measure the mass and volume of an irregular object.
• Accurately use laboratory equipment to gather data.
• Calculate the density of an irregular object using their data.
• Create a graph of mass vs. volume using class data and use the slope of the line to calculate the average density of the objects.
• Calculate the percentage error of their results using both the experimentally determined value and the accepted value for density.
• Create a lab report using tables and graphs, following a provided template.

Chemistry Topics

This lab supports students’ understanding of

• Density
• Measurement
• Data collection
• Significant digits and uncertainty
• Graphing and analyzing data
• Writing to communicate laboratory results.

Time

Teacher Preparation: 15-20 minutes (allow additional time to create wooden samples. See Teacher Notes below.)

Lesson:  1 hour for lab; 1 hour for lab report instruction and guided practice

Materials

• Several sets of irregular objects of varying materials (we use three different type of wood: Poplar, Plywood and Treated Southern Pine)
• Regularly shaped objects (blocks or cylinders) made from the same materials/wood as the irregular objects for calculation of accepted density values (if desired for extension of lab activity.)
• Rulers
• Graduated cylinders of appropriate size for irregular objects
• Laboratory balances
• Water

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• This lab is recommender for 1^st year high school chemistry classes.
• When creating the wood cut-outs for the first time, allow 30-45 minutes to layout and cut samples from each type of wood.
• This lab is designed to be a guided inquiry lab, as well as an introduction to lab report writing, as discussed in the corresponding article "Tools and Strategies for Lab Report Writing." We have students create their own lab report, using the provided lab report template while completing this lab.

• Background: The density of a substance is an intensive property of matter that can be used to help identify a substance. It is defined as the mass of the sample of substance divided by its volume, and is often measured in g/cm³ or g/ml. Students may have begun studying this in elementary school. By middle school they should have a fairly good understanding that some substances like lead are denser than others like wood. This activity should help them further their understanding by measuring the mass and volume of several objects, each with a different density, and comparing their results with members of their class.
  
  
• Tips: As much as possible, let the students work directly from the lab protocol, and only answer questions if 1) nobody in the lab group can do so and 2) the answer does not appear in the protocol. There is a big element of “discovery” in this lab. Make certain there is both a straightedge ruler and graduated cylinder available. If they have trouble with the volume of the object, ask them if they ever talked about water displacement in middle school. Ask if it is important whether the objects are dry before measuring the mass or volume. Ask what the ± of the instruments should be, and how many significant digits are in each measurement. Plastic graduated cylinders may be a better choice than glass if they are available.
  
  
• Management: The most common problem is students not pushing an object with density less than water's down so the whole object is submerged. Ask them to show you how they are measuring the volume before they make their final measurement.
  
  
• Differentiation: For advanced level classes, provide a block or cylinder of each substance so they can geometrically calculate the volume and, after measuring the mass, calculate an accepted value of density. For all, use this as an opportunity to calculate the percent error. Demonstrate with numbers for a substance they don't have, and then in guided practice watch them do that calculation with their own data.