Is Dilution the Solution to Pollution?

Author: Jennifer Titterington

Length of lesson: 50 minutes

Description of the class: Build Your Dream Home

Name: Chemistry

Grade level: 9-12th

Source of the lesson:

Keira Young Benchmark Lesson #3 - http://uteach.utexas.edu/%7Egdickinson/PBI/PBISpring03/waterq/content/Lesson3.htm

Amy Wagner, modified for a high school class.

http://thechalkboard.com/Corporations/Dow/Programs/1998_NSTA/1998%20Lessons/unit798.html

TEKS addressed:

1(A) demonstrate safe practices during field and laboratory investigations

2(B) collect data and make measurements with precision

2(C) express and manipulate chemical quantities using scientific conventions and mathematical procedures such as dimensional analysis, scientific notation, and significant figures

2(D) organize, analyze, evaluate, make inferences, and predict trends from data

2(E) communicate valid conclusions.

The Lesson:

I. Overview

Students will investigate the measurements of parts per million and parts per billion, so they will have a conceptualization to build on when they learn about safe levels of chemicals in water.

II.Ý Performance or learner outcomes

Students will be able to:

… Prepare serial dilutions and calculate their concentrations

… Define parts per million and parts per billion

… Evaluate the usefulness of dilution in eliminating chemicals from water.

III: Safety considerations

The teacher should not let the students drink their pollution solutions.

Although not harmful, the dye probably should not be ingested.

IV. Resources, materials and supplies needed

… Colorimeter with 6 cuvettes

… TI CBL

… TI-83 Calculator

… Red food coloring

… 6 beakers, 100 mL per group

… 10 mL graduated cylinder per group

… 100 mL graduated cylinder per group

… Stirring rod per group

… 1 Copy of data chart for each student

… 10 % solution (dye) for each lab group

… 1 eye dropper for each lab group

… 2 beakers of water for each lab group

Five-E Organization

Teacher Does Student Does

Engage:

Bring water tainted with trace chemicals. Ask the students to discuss what could be in the water. What units would they use?

Questions:

Has anyone heard of parts per million or parts per billion?

Do you know which is the greater concentration?

Hoped for Student Response:

Students will recognize that the water can be dangerous to drink, even though it is clear. Students will have heard of parts per million and parts per billion.

Expected Student Response:

Students may not realize that the water could be dangerous. Students may have heard of parts per million or parts per billion, but will not know which is bigger or what they are used for.

Evaluate

Were students able to realize that the water could be contaminated? If not, bring up Erin Brokovich, and similar occurrences. Students will explore the other concepts in the next exercise.

Explore:

Pass out the diagram and Instruction sheet to aid in explanation. Tell the students to fill a 100mL beaker with their 10% solution. They will then add 10ml of the original solution and add 90mL of distilled water to a new beaker. They will add 10ml of the solution and then add 90ml of distilled water in a new beaker. They will repeat this until they have six beakers of water.

Tell the students that they can calculate the concentration for each sample by taking the original percentage and multiplying by the dilution factor (1/10^th).

They will then use the colorimeter to determine percent transmittance. For every dilution, they will record the dilution factor, concentration expressed in parts, % concentration, concentration exponent, percent transmittance, and color of each solution.

Let them begin the experiment and go around to make sure that they all are on task and so that they may ask questions that arise.

Questions:

What is happening?

Does dilution mean that the concentration is becoming greater or less?

Hoped for Response:

Students will understand the procedure and complete the activity.

The students should complete the experiment portion of the lab and realize that the color of the solution is becoming paler and more dilute.

Students will recognize that they started out with a 100% solution but they may need help realizing that the dilution faction was 1/10. So they concentration is now 10%.

Expected Student Response:

Students will recognize that they started out with a 100% solution but they may need help realizing that the dilution faction was 1/10. So they concentration is now 10%.

Students will find the dilution, when prompted to do so.

Students will recognize that the solution is becoming more dilute and thus paler. As concentration decreases, % transmittance will increase.

Evaluate

Were the students able to complete the activity? Check for understanding while monitoring class work and prompt students to complete the task in a timely manner.

Explain:

Explain that one part per million is the same as .000001

Or one penny in $10,000, one minute in two years, or one dime in a one-mile-high stack of pennies.

Explain that one part per billion is the same as .000000001

Or, one penny in $10,000,000, one pinch of salt in 10 tons of potato chips, or one second in 32 years.

Questions:

What happened to the concentration each time?

What happened to % transmittance each time?

What does parts per million mean?

What does parts per billion mean?

Which is smaller?

At which concentration could you no longer detect the red food coloring with your eyes?

Did the colorimeter enable you to detect a difference between each concentration? Explain.

Hoped for Student response:

Students should notice that the dilution factor is always 1/10 and that you go from 100% to 10% to 1% and so on until they reach one part per million.

They should realize that % transmittance increased with increasingly dilute samples.

They should be able to answer all the questions.

Expected Student Response:

Students should all we writing out and solving the equation, when prompted to do so.

Students should notice that the dilution factor is always 1/10 and that you go from 100% to 10% to 1% and so on.

They should be able to answer all the questions.

Evaluate

Were the students able to recognize that parts per million is larger than parts per billion, proceed. If they are not, go over the concept of ratio and dividing into parts, 1/6 is less than ‡ etc. Compare sharing pizza, etc. Were the students able to explain the differences in detection between the colorimeter and the naked eye? If students seem to be experiencing great difficulty, a new lesson may be necessary.

Extend / Elaborate:

Give the students a table of concentration of the chemicals in the Survivor water sample as well as a table with their acceptable values. (Some will be very high, others lower concentrations). Most would have chosen this water sample as their safest bet on the first day’s activity and might be surprised to find out what is inside.

Have the students then calculate how much water they would have to add to dilute their water to a safe level. They should define parts per million and part per billion. Then, they should critically evaluate the effectiveness of dilution in eliminating contaminants.

Hoped For Student Response

Students will recall the definitions, the procedure for calculating dilution, and be able to critically evaluate dilution as a solution for each different chemical.

Expected Response:

Look over their results in order to apply them to the new calculations and then evaluate dilution as a solution..

Evaluate

Students should have realized that for some compounds, dilution would be safe and reasonable, but for some of the chemicals very small amounts would be toxic and dilution would be impossible. They should also recognize that dilution only works if you have enough pure water to dilute with. If students are unable to complete the assignment, give them some more real life examples Chemical Plants dumping in streams that exceeded acceptable levels. Then have calculation dilution factors for new data, but this time put a limit on the amount of water available for use with dilution.

Serial Dilution Instructions:

Label six beakers or cups with numbers 1 through 6.Ý Measure 90.0 mL of distilled water into each of the beakers.

Fill a cuvette to the line with concentrated food coloring. This concentration will be called 1/1, one part per one.
Into beaker #1, accurately measure 10.0 mL of concentrated food coloring and stir.
Carefully measure 10.0 mL of beaker #1 and stir into beaker #2.
Continue until you have stirred 10.0 mL of beaker #5 into beaker #6.Ý
Before continuing with the CBL colorimeter, complete the data table except the percent transmittance column. Be sure to observe the color with a white background behind the beaker.
Connect the colorimeter to the CBL with a CBL-DIN adapter in Channel 1, and link the TI-83 calculator to the CBL with a link cable. Turn on the CBL and the calculator. Press the PRGM key on the calculator and select the CHEMBIO program. Follow the prompts on the calculator to collect data for the experiment.
Perform a two-point calibration (0% and 100% transmittance) for the colorimeter and CBL with the TI-83. Use the blue LED (470 nm).

a) First, close the lid of the colorimeter, set the colorimeter knob to 0% T, and allow the reading on the CBL to stabilize. Press the [Trigger] button on the CBL and enter 0 when asked to Enter Reference.

b) Set the knob on the colorimeter to 470 nm and insert a blank cuvette (containing distilled water only). Allow the reading on the CBL to stabilize and press the [Trigger] button on the CBL.Ý Enter 100 at the Enter Reference prompt.

Fill each cuvette to the line with successive dilutions, keeping them in order for easy identification. Use good technique and carefully wipe the outside of the cuvette with soft tissues or paper towels before inserting the cuvette into the sample holder. To use the calculator to store the data, use the trigger/prompt mode on the calculator program and enter the concentration of each solution when requested. Note: The calculator will store concentration in List 1, absorbance in List 2, and percent transmittance in List 3.

Dilution Data Chart

Final Concentration = Original Concentration X Overall Dilution Factor

Beaker #	Overall Dilution Factor	Concentration (in parts)	% Concentration	Concentration Exponent	% Transmittance	Color
1
2
3
4
5
6