Grades: 9-12 (Science, Chemistry and Biology)
Time: 60 minutes
Space Requirement: Classroom with sink and ample counter space
Methodology: Hands-on and Cooperative Learning, Group Discussion
Materials not included in the kit:
For each group: Regular black (orange pekoe) tea bag, a watch or timer
For the Teacher: Small amount of chlorine bleach, beaker (at least 100mL)
Materials included in the kit that will be used:
For each group: Total Chlorine test strips, 2 plastic cups, empty 5mL vial
For the Teacher: Pipette
Objectives: Students will expand their understanding of chlorine demand by participating in an experiment. Students will learn about the Guidelines for Canadian Drinking Water Quality guideline for total chlorine. Students will learn how to perform calculations involving concentration, volume and dilution factors.
Directions/Procedure:
- Immediately before the class, prepare the chlorine solution by following the directions found in the “For the Teacher” section below. This will not take longer than a minute.
- Distribute printed copies of the Lesson Two Handout to the class. Read through the first part with the class and then have each student carefully read the instructions for the activity.
- Have the class split into their groups.
- Separate the materials listed for this activity from the rest of the kit and have someone from each group collect what they need.
- Each group will work through the chlorine demand experiment.
- Once each group has the results from their test, all extra water from your chlorine solution and the tea bag experiment can be disposed of down a sink. The plastic cups are not to be thrown away; they should be rinsed and reused in later experiments.
- Ask the students if, when they tested the tap water for chlorine in the previous lesson, they got appropriate results of over 0.5mg/L. A chlorine concentration under 0.5mg/L in tap water can be a real problem that should be addressed by a representative from your local water treatment facility. This would be a good topic to bring up if you can arrange a meeting with such a representative.
- Ask the students how they think the results of this experiment are significant to the water treatment process. You can lead them with questions such as: How did the chlorine demand from the tea bag affect the chlorine concentration of your water sample? How might tap water be affected if it were exposed to a similar chlorine demand either before or during treatment or in the distribution system? Students should identify that this experiment indicates that maintaining the minimum chlorine concentration can be difficult. If a seemingly small amount of contaminant, such as that provided by the tea bag, can use up much of the chlorine in a water sample then a source of contamination in treatment and distribution systems can do the same. You could also ask them to think about the balancing act of needing to add more chlorine to meet the chlorine demand while not adding so much that chlorine byproducts begin to build up.
- An optional activity for a class that includes more advanced calculations is to demonstrate (or ask students if they can demonstrate) how the total chlorine concentration of the solution you prepared can be found. A guide to performing this calculation can be found in the “For the Teacher” section below.
Evaluation: Students can be evaluated based on their participation and on how they cooperate with their fellow group members.
For the Teacher:
Preparing the Chlorine Solution
Put 100mL of tap water into a beaker. You will need some chlorine bleach (look for 4% sodium hypochlorite on the label). Using the pipette, put 4 drops (about 0.12mL) of bleach into the beaker and then mix this solution. Keep the pipette to use again in Lesson 4. This process only takes a minute and should be done just before the solution will be needed. Each group will need 5mL of this solution which they will be diluting further.
Calculating the Total Chlorine Concentration of the Prepared Solution
First you need an estimate of the amount of water in the students' cups when they added water to the 5mL of chlorine solution. Ask students how they might find this volume now that the cup has been poured out. The best ways are to refill the cup as close as you can to where it was and then either pour it into a graduated cylinder or beaker and read the volume off or to weigh the cup and use the known density of water to find the volume.
We will say that it is 80mL in this example: Adding 80mL of tap water to the 5mL of prepared sample results in an 85mL solution. Dividing this volume by 5mL gives the factor that your solution was diluted by so you can say that by adding the 5mL to 80mL you diluted it 85/5 = 17 times. This means that the total chlorine concentration of the solution you prepared is about 17 times greater than the total chlorine concentrations that the students found before adding the tea bag.
In a more advanced class or for higher grade levels you may wish to do the calculation while taking into consideration the chlorine contribution from the tap water.
This more advanced calculation can be done by first finding the mass of the chlorine in both the initial sample of tap water and final mixed solution. To find the mass of the chlorine in the tap water sample you multiply the chlorine concentration of your tap water by the volume of the water in the cup. The mass of the chlorine in the mixed sample before the tea bag was added can be found by multiplying the chlorine concentration of that sample by its volume. Do not forget that unit conversions will be necessary. The mass of the chlorine in the students' 5mL samples of the solution you prepared can be found by simply subtracting the initial chlorine mass from the final chlorine mass. Finally, the concentration of the prepared chlorine solution can be found by dividing this mass by 5mL.
Related Links:
What is Chlorination? Fact Sheet
Operation Water Biology
Lesson Two
Chlorine
More information on chlorination can be found at: www.safewater.org/fact-sheets-1/2017/1/23/what-is-chlorination
Since chlorine gets used up when removing contaminants from water, the more contaminants there are the more chlorine is needed to get rid of them. The total amount of chlorine that must be added to water to fully disinfect it is known as that water’s chlorine demand. For good sources of water the chlorine demand is low and the water treatment facility only has to add a small amount of chlorine to disinfect the water and make it safe to drink. Some extra chlorine is usually added to make sure that no bacteria can grow in the pipes and distribution system and that the water is still clean when it comes out of your tap. In fact, unlike most of the other water quality guidelines which state the maximum recommended amount of a contaminant in your drinking water, the guideline for chlorine is the minimum allowed amount. Any time that all of the chlorine gets used up before the water comes out of the tap there is a risk that bacteria could begin to grow in the water pipes. The current guideline for total chlorine from the Guidelines for Canadian Drink Water Quality is a minimum concentration of 0.5mg/L. Did your tap water meet this requirement when you tested it in the previous lesson?
It is easy to show the effect that a contaminant can have on the total chlorine concentration of a sample of water. In this experiment you will see that the chlorine demand created by a little bit of tea is enough to use up all of the chlorine in a water sample.
Materials needed for this experiment are:
Two total chlorine test strips
Two plastic cups
Tea bag
Empty 5mL vial
Watch or timer
1. Use the 5mL vial to collect 5mL of the chlorine solution that your teacher has prepared.
2. Pour this sample into a cup then fill that cup about 3/4 of the way up with tap water to dilute the solution. Swirl the water around a little bit to make sure it is well mixed.
3. Fill the second cup about 1/4 of the way up with this diluted chlorine solution. The first cup should still be about half full; set it aside for now and perform a total chlorine concentration test on the sample in the second cup.
Initial total chlorine concentration of chlorine solution:
_________________________
4. Once you have the result from that test you should empty and rinse the second cup.
5. Dip the tea bag into the first cup two or three times until the water just begins to turn colour. Do not let the tea bag sit in the water. If the water changes colour too much it could affect the colour of the test strip when you do the test. The tea bag can be thrown away or composted.
6. Do a total chlorine concentration test on the sample that you dipped the tea bag into. When you have the results from the test all the water can be disposed of. The cups should be rinsed and kept.