Summary

In this lab, students will experimentally determine the density of several liquids and solids from mass and volume measurements. They will then identify an unknown metal by comparing its density to that of known metals and calculate the percent error within the class for a specific sample.

Grade Level

Middle School, High School

NGSS Alignment

This lab will help prepare your students to meet the following scientific and engineering practices:

• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data

Objectives

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

• Calculate the density of liquids and solids from mass and volume measurements.
• Measure the volume of a solid using the displacement method.
• Identify an unknown substance by determining its density and comparing it to densities of known substances.
• Calculate percent error.
• Explain if results are accurate and/or precise.

Chemistry Topics

This lab supports students’ understanding of:

• Density
• Measurement
• Significant figures
• Percent error
• Error analysis

Time

Teacher Preparation: 20 minutes
Lesson: 60 minutes

Materials

For each group:

• 25-mL and 10-mL graduated cylinder (preferably plastic)
• Small pieces of metal and plastic (such as these density cylinders or these density cubes)
• Balance (0.001 g if possible)
• Water
• Pepsi (about 10 mL)
• Diet Pepsi (about 10 mL)
• M&Ms (5)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical, not for consumption.
• 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 is a good activity to use early in the school year when teaching measurement, significant figures, and, of course, density. Students will practice measuring masses and volumes using balances and graduated cylinders with various liquids and solids, including familiar materials such as water, soda, and M&Ms.
  • Remind students not to eat their M&Ms or drink the Pepsi during lab!
• Prior to completing this lab, students should have some understanding of measurements and significant figures, particularly regarding recording measurements with one estimated digit and rounding for their density calculations. (If using with middle school, you could just tell them how many decimal places to round to in the calculations.)
  • The Measuring Volume Simulation in the AACT resource library is a good way to remind students how to record volume measurements to the correct number of digits.
  • Milligram balances (0.001 g) are preferred, as they will allow students to use more significant figures, but centigram balances (0.01 g) will also work.
• The prelab asks students to read the procedures and create appropriate data tables. If time is limited or students would struggle with this task, you could develop the data table as a whole class, or you could provide students with a data table (see answer key).
  
• It is strongly recommended that students use plastic graduated cylinders for measuring the volume of the metal samples, not glass cylinders. If students use glass cylinders and drop the metal samples in too quickly, they may break bottom of the cylinder off.
• Students should notice a difference in density between regular and diet soda – the amount of sweetener by mass is much greater in regular soda (containing sugar) compared to diet soda (containing smaller amounts of artificial sweeteners). Consider pairing this lab with the Sinking Soda demo, either before or after, to help them draw the connection between their calculations on paper and their observations in real life.
• For more practice, you could add other materials or objects for students to measure, or have students bring in their own additional samples. They could compare other drinks (juice, energy drinks, tea/coffee, etc.), types of candies, or even different types of M&Ms (minis, peanut, etc.).
• Science education companies often sell sets of samples of the same size/shape (but different masses and therefore densities) for density investigations. For example, Flinn Scientific offers this set of cylinders or this set of cubes that could be used for the unknowns in this activity.
  • If using regularly shaped samples, such as the cylinders or cubes in the links above, consider having students compare their volume by displacement measurement to a calculated volume using length measurements and geometrical formulas for the volumes of regular objects.
  • If you do not have a set of density cylinders or cubes, other samples could also be used, such as various metal wires, shot, strips/sheets, etc. that you already have in your lab.
  • Do not tell students beforehand what metal they are working with – they should be able to determine its identity from their lab data.
  • You could provide students with a list of metals and their densities, or you could have them do research on their own to try to identify their unknown metal by its density. If students do their own research, they may have a hard time identifying their metals if there are multiple metals with similar densities, or if their metal samples are impure or oxidized – this can be a good opportunity to discuss sources of error and other properties or methods for identifying unknowns.
  • Similar to the metal unknown, you could also have students try to identify the plastic sample (ex: acrylic, nylon, etc.) by comparing their results to published density data.
• In our trials, the density of M&Ms came out to 1.33 g/mL. You may provide this as the accepted value – alternatively, you could use the class average, or you could calculate the density ahead of time by following the procedures in Part III. If you do your own density calculations, you could use more M&Ms to lessen the impact of outliers and get a more accurate value.
  • If you would prefer to have your students use a more standardized measurement for their percent error calculations, you could have them complete the percent error calculation for water or for their unknown metal after they have identified it, rather than M&Ms.
• Classroom resources from the AACT Library that may be used to further teach this topic include:
  • Simulation: Density
  • Animation: Density
  • Unit Plan: Chemical Measurements Unit Plan
  • Lab: Bowling Ball – Will it Sink or Float in Water?
  • Demonstration: Sinking Soda
  • Lab: Significant Figures and Lab Data
  • Lab: You Are My Density
  • Activity: Identifying Plastics with Density Data