Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Using a wax pencil or dry erase marker, label three beakers A, B and C.
2. Add approximately 500 mL of tap water to each beaker.
3. Add 10 drops of red dye solution to beaker A.
4. Add 10 drops of yellow dye solution to beaker B.
5. Add 10 drops of blue dye solution to beaker C.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The waste solutions may be disposed of according to Flinn Suggested Disposal Method #26b.

Teacher Tips

• This is a great teacher-guided cooperative lab activity. Perform the demonstration in front of the class and have a different student or student group perform each step.
• As an extension, ask students to develop their own scenarios for color mixing and dilutions and have them exchange their procedures with another group to test their write-ups. Green dye has been included for use in this extension activity.

Sample Data

Answers to Questions

1. Summarize the results of this activity in a few sentences.

   A rainbow spectrum of colored water—red to blue—was observed, with approximately equal volumes of liquid in each tube.

2. Re-read the demonstration instructions above. Predict the volume of water that should have been obtained in each tube and the color of each resulting solution.

   Each tube should have had an identical volume of liquid (15 mL) and the colors should have been a rainbow spectrum—red, orange, yellow, green and blue.

3. What are likely sources of experimental error in this activity? Describe how they would have affected the results.

   Errors in measuring precise volumes of liquid are most common, and would lead to variation in the amount of liquid in each tube at the end. The other errors are due to residual solution in the graduated cylinders and poor mixing, leading to “blended” colors as opposed to clean red, orange, yellow, etc.

4. A student measured 25 mL of water in a small beaker and transferred it into a graduated cylinder. The volume was 25.6 mL. Explain in terms of accuracy and precision.

   Beakers are neither accurate nor precise for measuring exact volumes because typical markings may be only every 10 mL. The “25 mL” was an estimated volume. Graduated cylinders typically have markings every 0.1–1 mL, depending on the size of the cylinder and thus can be measured with greater precision and the result is likely more accurate, representing the true volume.

Discussion

Much of what we know about the physical world has been obtained from measurements made in the laboratory. Skill is required to design experiments so that careful measurements can be made. Skill is also needed to use lab equipment correctly so that errors can be minimized. At the same time, it is important to understand the limitations of scientific measurements.

Accuracy and precision are two different ways to describe the error associated with measurement. Accuracy describes how “correct” a measured or calculated value is, that is, how close the measured value is to an actual or accepted value. The only way to determine the accuracy of an experimental measurement is to compare it to a “true” value—if one is known! Precision describes the closeness with which several measurements of the same quantity agree. The precision of a measurement is limited by the uncertainty of the measuring device. Uncertainty is often represented by the symbol ± (“plus or minus”), followed by an amount. Thus, if the measured length of an object is 24.72 cm and the estimated uncertainty is 0.05 cm, the length would be reported as 24.72 ±0.05 cm.

Variations among measured results that do not result from carelessness, mistakes or incorrect procedure are called experimental errors. Experimental error is unavoidable. The magnitude and sources of experimental error should always be considered when evaluating the results of an experiment.

Introduction

This simple cooperative classroom demonstration illustrates the importance of accurately following laboratory procedures in order to achieve reproducible results.

Concepts

• Scientific method;Measurement;Accuracy;Experimental error

Experiment Overview

In this activity, different amounts of colored solutions will be added to or removed from a series of demonstration tubes and the results studied to determine the accuracy of measurements and if directions were followed.

Safety Precautions

Dye solutions will stain skin and clothing. Wear chemical splash goggles and avoid contact of all solutions with eyes and skin.

Procedure

1. Obtain five demonstration tubes, demonstration rack, three beakers of colored water (A–C), and three graduated cylinders.
2. With a wax pencil or dry erase marker, label the tubes 1–5 from left to right (see Figure 1).
   {12425_Procedure_Figure_1}
3. Recruit seven student volunteers to perform steps 4–10.
4. Measure 25 mL of solution from beaker A using a graduated cylinder. Pour into tube 1.
5. Measure 28 mL of solution from beaker B using a clean graduated cylinder. Pour into tube 3.
6. Measure 22 mL of solution from beaker C using a clean graduated cylinder. Pour into tube 5.
7. Pour 8 mL of solution from tube 3 into a graduated cylinder and add it into tube 4.
8. Pour 7 mL of solution from tube 5 into a graduated cylinder and add it into tube 4. Mix the solution by swirling the tube.
9. Pour 10 mL of solution from tube 1 into a graduated cylinder and add it into tube 2.
10. Pour 5 mL of solution from tube 3 into a graduated cylinder and add it into tube 2. Mix the solution by swirling the tube.
11. Record observations and answer questions on the Measurement and Accuracy Worksheet.

Student Worksheet PDF

12425_Student1.pdf