Introduction
How do you convey the amount of material remaining when an environmental contaminant is present in one part per thousand or one part per million? Make the concept of serial dilution more concrete for your students with this unique modeling activity. Simulated process of serial dilution results in a colorful model of diluted solutions.
Concepts
- Solutions
- Concentration
- Serial dilution
- Powers of 10
Experiment Overview
Blue and yellow beads will be placed into demonstration tubes and used to simulate and model the process of serial dilution.
Materials
Beads, blue, 90 mL*
Beads, yellow, 2 mL*
Cup, plastic, 30-mL*
Demonstration tubes and caps, 5*
Marker, permanent
Rack, demonstration tube*
*Materials included in kit.
Safety Precautions
Follow all laboratory safety guidelines.
Procedure
{12429_Procedure_Figure_1}
- Obtain five demonstration tubes and caps, demonstration rack, 30-mL plastic cup and blue and yellow beads.
- Have students sketch the steps in the serial dilution procedure and record their observations on the Serial Dilution Activity Worksheet as the demonstration is performed.
- Place the five demonstration tubes in the demonstration rack.
- Place caps on all five of the demonstration tubes.
- Using a permanent marker, write the following ratios on the top of each cap from left to right (see Figure 1).
- Remove the caps from the tubes and place them on the tabletop next to the corresponding tubes.
- Measure approximately 18 mL of blue beads into the 30-mL plastic cup.
- Add the 18 mL of blue beads into the first demonstration tube. The blue beads represent the water used in the dilution.
- Repeat steps 7–8 for the remaining four demonstration tubes. Note: These steps may be done before beginning the demonstration if desired.
- There is a 2.5-mL mark on the plastic cup. Add enough yellow beads so that they are just below this 2.5-mL mark. The yellow beads represent 2 mL of a “stock” solution.
- Place the 2 mL of yellow beads into the first demonstration tube.
- Cap the tube and shake and swirl to thoroughly mix the beads. This tube represents the 1/10 dilution of the stock solution.
- Unscrew the cap of the 1/10 dilution tube and measure 2 mL of the “solution” into the 30-mL cup.
- Place the 2 mL of the 1/10 dilution into the 1/100 demonstration tube.
- Cap the tube and shake and swirl to thoroughly mix the beads.
- Repeat steps 13–15 consecutively using the 1/1000, 1/10,000 and 1/100,000 tubes in series.
- Have students answer the questions on the Serial Dilution Activity Worksheet.
Teacher Tips
- There are enough beads given in this kit to perform the activity as written more than 10 times.
- Allow students to shake and swirl the tubes to get a closer look at the ratios of blue and yellow beads in the tubes.
- To simplify the math reasoning, you may wish to just say that you are taking 1 mL of solution from each tube (instead of saying 2 mL is being taken) and that 9 mL of “water” (blue beads) are present in each tube (instead of 18 mL).
- Students may also perform their own serial dilutions using the beads provided in this kit and test tubes. It is recommended that students use 9 mL of blue beads and 1 mL of yellow beads for their dilutions.
- After performing the activity, the caps may be glued onto the demonstration tubes and the completed serial dilution tubes may be kept on display as a permanent model in your classroom.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Analyzing and interpreting data
Using mathematics and computational thinking
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Crosscutting Concepts
Patterns
Systems and system models
Performance Expectations
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
MS-ETS1-3. Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
MS-ETS1-4. Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.
Answers to Questions
- Record all observations and draw a figure of the steps involved in the serial dilution activity.
Student answers will vary. The yellow stock solution was diluted by a factor of 10 with each dilution.
{12429_Answers_Figure_2}
- What are the concentrations of the “solutions” in each tube compared to the original yellow bead stock solution?
From the first tube to the last tube (left to right): 1/10, 1/100, 1/1000, 1/10,000 and 1/100,000.
- Compare and describe the relative intensity of the yellow “color” in the dilution series. If you had used a “real” yellow solution in water, do you think you would still be able to see the color in the 1/100,000 tube? Explain.
The yellow color or number of yellow beads in the tubes became less and less as the dilution series was performed. The yellow color in a “real” serial dilution probably would not be seen in the 1⁄100,000 tube because it is at such a low concentration.
- Why is it important to shake and swirl each tube before transferring the “solution” to the next tube in the sequence?
Shaking and swirling each tube will ensure that the “solution” will become thoroughly mixed before each dilution.
- Water samples frequently must be diluted when analyzing bacterial contamination in order to conveniently count bacterial colonies. If a water sample was diluted twice by a factor of 10 to give a bacterial count of 25 colonies, what was the bacterial count in the original sample?
2500
- You are given a stock solution of 1.0 M potassium sulfate. How would you use a serial dilution to prepare a 0.001 M potassium sulfate solution using the stock solution? Describe and/or sketch the procedure below.
{12429_Answers_Figure_3}
Discussion
A serial dilution is a dilution sequence in which a series of solutions is prepared. In this case, each one is one-tenth as concentrated as the previous one. To prepare serial dilutions, 1 mL of a stock solution is diluted with 9 mL of water. Then 1 mL of the resulting solution is diluted again with 9 mL of water. This process is repeated until the desired concentration has been reached. Since each solution is 1⁄10 as concentrated, the concentrations can simply be divided by 10 down the line (see Figure 2). Note that in this activity, 2 mL of the stock bead “solution” and 18 mL of “water” is used to scale up the demonstration.
{12429_Discussion_Figure_2_Serial dilution}
For example, if a serial dilution was performed on a 0.10 M sodium chloride solution, the first dilution would be 0.010 M, the second 0.0010 M, the third 0.00010 M and so on. Serial dilutions are commonly used in microbiology where the solution being diluted contains bacterial colonies. It is important that the number of colonies growing in the solution not be too large, so bacterial solutions are commonly diluted down to concentrations of 1 in 1,000,000 (10 –6) or one part per million (ppm). The end result is a million times less concentrated than the original solution! The concepts of one part in a thousand (ppt) or one part per million (ppm) are frequently encountered in environmental science to describe the relative concentrations of constituents or impurities in air and water. The concentration of salt in seawater, for example, is 35 parts per 1,000 while the amount of carbon dioxide in air is one part per thousand. The standards for environmental contaminants are often expressed in parts per million (ppm) or parts per billion (ppb). The maximum allowable concentration of lead in drinking water is only 0.015 ppm (15 ppb)
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