Hands-on Activity Determining Concentration

Learning Objectives

After this activity, students should be able to:

• Explain how light reflection can be used to measure the concentration of a solution.
• Draw an approximate linear regression through a given set of data.
• Calculate percent change and percent error.
• Use a set of standard solutions to determine information about the concentration of unknown solutions.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• LEGO MINDSTORMS Education EV3 Core Set (includes color sensor) available at https://education.lego.com/en-us/products/lego-mindstorms-education-ev3-core-set/5003400#lego-mindstorms-education-ev3. If not enough LEGO bricks and sensors are available for each group, conduct the data gathering portion of the activity as a class demonstration, sharing the reflected light data with the class, so students can complete the rest of the activity
• 9 plastic cuvettes, labeled A-G and 1-2, a pack of 100 available at https://www.universalmedicalinc.com/brandtech-brand-polystyrene-spectrophotometry-cuvettes.html?
• ruler
• Pre-Activity Assessment, one per student.
• Determining Concentration Worksheet, one per student
• Post- Activity Assessment, one per student

To share with the entire class:

• red liquid food coloring
• water
• 9 containers, to hold 20 ml of liquid each
• masking tape/pen or wax pencil, to label 9 containers

Note: This activity can also be conducted with the older (and no longer sold) LEGO MINDSTORMS NXT set instead of EV3; see below for those supplies:

• LEGO MINDSTORMS Education NXT Base Set (includes light sensor)

Worksheets and Attachments

Pre-Req Knowledge

Ability to calculate the percent difference between two numbers, and plot data on Cartesian graphs.

Introduction/Motivation

Have you ever made Kool-Aid or another flavored drink mix powder, such as lemonade or a sports drink? If so, you already know that you can change the strength or concentration of a solution (your drink) by adding more or less water. If you want a strong Kool-Aid drink (one that has deep color and flavor) you dissolve a lot of the powder in your glass of water, right? What about if you're in the mood for a drink that is more watered-down with just a hint of Kool-Aid sweetness? Well, then you only add a small amount of the powder to your glass of water. So in making a Kool-Aid drink to your liking, you are actually adjusting concentration, with more powder (per volume liquid) making the drink more concentrated, and less powder (per volume liquid) making the drink less concentrated. The opposite of concentrating is diluting, or in this example, adding more water.

Can you think of two ways you can approximate the concentration of a Kool-Aid drink? (Answers: Tasting it and making observations about the color. The more concentrated the drink, the more intense the flavor and the darker the color.)

Scientists and engineers have another accurate and straightforward way to study concentration using light—something you will get to experience today! In this activity you will study the concentration of red dye in different solution samples based on how much light they reflect. In order to do this, we will use liquid solutions that are transparent, or allow light to pass through them. The more light that a solution is able to reflect, the more concentrated it is! We'll evaluate a set of standard solutions with known concentrations of red dye and put your math and science skills to work in order to determine the dye concentrations for two unknown solutions.

Procedure

Background

When light is shone on a solution, any particles or small molecules in the sample disrupt the light's originally straight path of travel. As a result, the light "scatters," meaning it bounces off the particles and travels in all directions. The higher the concentration of particles in a sample, the more light scatters.

Before the Activity

• Gather materials and make copies of the Pre-Activity Assessment, Determining Concentration Worksheet, Post-Activity Assessment, one each per student.
• Prepare seven standard solutions (see A-G below) and two solutions for "unknown" samples (see 1 and 2 below). Begin by pouring 20 ml of water into each of the nine containers. Label the containers (A-G and 1-2) and prepare each of the solutions as follows:

A. Add 50 drops red food coloring to 20 ml water

B. Add 30 drops red food coloring to 20 ml water

C. Add 20 drops red food coloring to 20 ml water

D. Add 10 drops red food coloring to 20 ml water

E. Add 5 drops red food coloring to 20 ml water

F. Add 1 drop red food coloring to 20 ml water

G. 20 ml water

1. Add 15 drops red food coloring to 20 ml water

2. Add 40 drops red food coloring to 20 ml water

• Organize 9 cuvettes for each student group. Add approximately 2 ml of each solution to the cuvettes. Each group of students should have a complete set of solutions (A-G and 1-2), one solution in each cuvette (see Figure 1).

  

• Prepare for each group one EV3 brick, equipped with a color sensor and a cable to connect the sensor to the brick.

With the Students

1. Administer the pre-activity assessment.
2. Present the Introduction/Motivation information to students.
3. Divide the class into groups of four students each.
4. Distribute to each group one EV3 brick with the color sensor and cable, a set of 9 solutions and worksheets (one per student).

   

   Explain and demonstrate the correct set-up for taking "Reflected Light (%)" measurements (the values that students need to record as data in their worksheet tables). Have students position their color sensors and cuvettes on a desk or table (see Figure 2), making sure that nothing is behind the cuvettes (including their hands). If students want to hold the cuvettes while taking the measurements, they may do so by pinching them at the top, so as to prevent any interference from their hands. Remind students that:

• It is important to perform the measurements for all solution samples in exactly the same manner to reduce experimental error.
• If student hands are behind or beside the cuvettes when measurements are taken, the light may reflect off their hands and give inaccurate light value readings.
• Avoid taking measurements in direct sunlight, since ambient light will affect the values.

6. Guide students through the following steps to operate their EV3 bricks:

• Turn on the EV3 brick by pressing the center button. After you press the button, the Brick Status Light should turn red and the Starting screen should display. When the light changes to green, your EV3 Brick is ready.
• Press the right brick button twice to get to the "Brick App" tab. You will see the first app named "Port View" which is highlighted by default. Press the center button to enter the port view.
• Press the right brick button twice to see the Port 3 value which should display "3: COL-REFLECT" and then a percentage value. Thus the amount of reflected light (expressed in percent) appears on the EV3 display screen. 

7. Direct students to record in the worksheet table the reflected light value that is displayed when the sensor is held up against each solution sample, as shown in Figure 3.
8. Give students time to measure and record reflected light values for all standard solutions and both unknown solutions.
9. After students have collected and recorded data for all samples, direct them to plot their data points on the grid provided on their worksheets. Put reflected light values on the y-axis, and concentration on the x-axis.
10. Have students use a ruler to draw a straight line through as many of the plotted points for the standard solutions as they can. Explain that in order to draw a line that best fits the data, they should look at all data points and line up the ruler such that some of the points fall above the line, and some below. Then draw a single line that that passes through the middle of the points.
11. Next, guide students through determining the concentration values for the unknown solutions by using the line plotted for the standard solutions. Direct students to:

• Locate the reflected light value for Unknown 1 on the y-axis.
• Match the value on the y-axis to its position on the standard solutions line, and record the corresponding concentration.
• Repeat for Unknown 2 and record both concentration values in the worksheet table.

12. Give students time to complete the following on their worksheets:

• Determine the percent change (using the equation provided) between the reflected light value of Standard A and Standard D.
• Determine the percent change (using the equation provided) between the concentration of Standard A and Standard D.
• Compare the two calculated percent change values they calculated and make observations about the values.

13. Write the actual concentrations for Unknown 1 and Unknown 2 on the board. The actual concentration for Unknown 1 is 15 drops/cuvette, and the actual concentration for Unknown 2 is 40 drops/cuvette.
14. Instruct students to calculate the percent error for the unknown solution concentrations they determined using the equation provided.
15. Assign any worksheet questions not completed during the class period as homework.
16. Administer the post-activity assessment.

    

Vocabulary/Definitions

concentration: The amount of a component in a given area or volume.

dilute: To reduce concentration by adding more liquid.

incident light: Direct light that falls on a surface or a substance.

insoluble: Incapable of being fully dissolved into a liquid.

media: A liquid solution.

reflected light: Incident light that bounces off a substance in the direction from which it came.

scatter light: When incident light bounces off a substance in many different directions.

soluble: Capable of being dissolved into a liquid.

standard: Something set up and established as a rule for the measure of quantity, weight, extent, value or quality.

transmit light: When incident light travels through a substance.

unknown: Something that requires discovery, identification or clarification.

Assessment

Pre-Activity Assessment

Pre-Assessment: Before starting the activity, administer the five-question Pre-Activity Assessment to introduce the topic and gauge students' baseline knowledge of the subject matter.

Activity Embedded Assessment

Worksheet: Direct students to complete the Determining Concentration Worksheet as they conduct the activity, collect and plot data, make calculations, and answer questions. Have students complete their own worksheets, but encourage them to collaborate with their group members. Remind student groups not to share answers with one another.

Post-Activity Assessment

Post-Assessment: At activity end, administer the Post-Activity Assessment. Review student answers to gauge their concluding comprehension of the subject matter.

Troubleshooting Tips

Remind students that the cuvettes have no lids, and they should be careful not to spill their solutions.

Activity Scaling

• For lower grades, eliminate the data plotting and simply have students record the values in the table.
• For upper grades, have students use Microsoft Excel to plot their data and perform regression analyses on the set of standard solutions. Have students determine the equation for the standard solutions curve and use it to generate the concentrations of the unknown solutions. Also have students estimate percent reflected light for the unknown solutions using the known concentration values, as well as calculate percent error.

Copyright

Contributors

Supporting Program

Acknowledgements

This activity was developed by the Applying Mechatronics to Promote Science (AMPS) Program funded by National Science Foundation GK-12 grant no. 0741714. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.