Hands-on Activity: Determining Concentration

Contributed by: AMPS GK-12 Program, Polytechnic Institute of New York University

A photograph shows a student at a table using a LEGO robot to shine light from a small device into orange liquid in a vial.
Students use color sensors to measure the percent of light reflected from a solution.
copyright
Copyright © 2013 Jasmin Hume, Polytechnic Institute of NYU

Summary

Students quantify the percent of light reflected from solutions containing varying concentrations of red dye using LEGO© MINDSTORMS© EV3 bricks and color sensors. They begin by analyzing a set of standard solutions with known concentrations of food coloring, and plot data to graphically determine the relationship between percent reflected light and dye concentration. Then they identify dye concentrations for two unknown solution samples based on how much light they reflect. Students gain an understanding of light scattering applications and how to determine properties of unknown samples based on a set of standard samples.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

Many types of engineers, such as chemical, biomedical and environmental engineers, often need to know information about solution properties (such as concentration) that is difficult to determine from visual observation alone. Regulating the concentration of various chemical compounds, whether soluble or insoluble, is important to chemical engineers working on large-scale production of medicines, food, oil and gas, and virtually every liquid you can think of. In another example, biosensors (designed by biomedical engineers) must be highly sensitive in order to produce accurate measurements of low biomolecule concentrations in biological fluids, such as blood and saliva. The technique of measuring concentration in a liquid using reflected light or light scattering is widespread in engineering industry applications, as it provides quick, convenient and fairly accurate concentration measurements in a process that can be automated with the help of robotics.

Pre-Req Knowledge

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

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.

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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.

  • Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use parentheses, brackets, or braces in numerical expressions, and evaluate expressions with these symbols. (Grade 5) Details... View more aligned curriculum... Do you agree with this alignment?
  • Find a percent of a quantity as a rate per 100 (e.g., 30% of a quantity means 30/100 times the quantity); solve problems involving finding the whole, given a part and the percent. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently divide multi-digit numbers using the standard algorithm. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • New products and systems can be developed to solve problems or to help do things that could not be done without the help of technology. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use parentheses, brackets, or braces in numerical expressions, and evaluate expressions with these symbols. (Grade 5) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently divide multi-digit numbers using the standard algorithm. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Find a percent of a quantity as a rate per 100 (e.g., 30% of a quantity means 30/100 times the quantity); solve problems involving finding the whole, given a part and the percent. (Grade 6) Details... View more aligned curriculum... Do you agree with this alignment?
  • Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
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Materials List

Each group needs:

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)

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.

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.

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

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).
    Photograph of a set of 9 standard solutions labeled A-G.
    Figure 1. Standard solutions made of varying amounts of red food coloring and water.
    copyright
    Copyright © 2013 Jasmin Hume, Polytechnic Institute of NYU
  • 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).
    A photograph shows a hand holding a plastic LEGO device with one end touching a plastic container (shaped like a squared-off test tube) of red liquid.
    Figure 2. To measure reflected light of a solution, position the color sensor on a flat surface directly in front of the solution sample, with the clear side of the cuvette facing the color sensor.
    copyright
    Copyright © 2017 University of Colorado Boulder
    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.
  1. 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. 

A light reflectance value shown on the LEGO EV3 brick
Figure 3. The reflectance value appears on the display panel of the LEGO EV3 brick.
copyright
Copyright © 2017 University of Colorado Boulder

  1. 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.
  2. Give students time to measure and record reflected light values for all standard solutions and both unknown solutions.
  3. 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.
  4. 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.
  5. 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.
  1. 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.
  1. 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.
  2. Instruct students to calculate the percent error for the unknown solution concentrations they determined using the equation provided.
  3. Assign any worksheet questions not completed during the class period as homework.
  4. Administer the post-activity assessment.
    Photograph of students recording data and plotting values to determine the concentrations of unknown solutions.
    Concentrations of unknown solutions are determined by plotting reflectance values for standard solutions. Students collect, plot, and analyze the data using robots and color sensors.
    copyright
    Copyright © 2013 Jasmin Hume, Polytechnic Institute of NYU

Attachments

Troubleshooting Tips

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

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.

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.

Contributors

Jasmin Hume

Copyright

© 2013 by Regents of the University of Colorado; original © 2013 Polytechnic Institute of New York University

Supporting Program

AMPS GK-12 Program, Polytechnic Institute of New York University

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.

Last modified: August 18, 2017

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