Time frame: 2 class periods

Introduction:

Students often hear the expression parts per million when discussing issues in the environment and in science, but until they have had the opportunity to experience the scale of these numbers first hand these concepts can be hard to grasp. In this lab students will mix solutions and then be asked to try to detect small concentrations of lemon juice and food coloring using their own senses.

The concepts explored in this activity - dilution, quantity, size, indicators and detectabilty are critical foundation pieces to understanding databases and impacts of numbers in the real world.

Purpose:

The purpose of this activity is to explore dilution and detection through measuring mixing and sensing.

Materials

strips of white paper to fit around test tubes test tubes / rack tape index cards for labelling lemon juice distilled water food coloring pH sensors data sheet / pencil graduated cylinders computer and the internet for part B - or printed copies of the web pages

Part A: Indicators, Perception and Measurements

1. In this section of the lab students will be tracking the presence of two substances - lemon juice and red food coloring - diluted in water.
2. How can these be detected? Design your ideas for detection with your group. List them in your report.
3. Label your containers (test tubes) with a secret code and create a key that shows both the percent solution and how many parts lemon juice or food coloring each contains. You will have 6 containers. Label them and create your code before beginning your dilution. use a pencil to write on the tubes.
4. the dilution are 1/10, 1/100, 1/1000, 1/10,000, 1/100,000, 1/1,000,000
5. Begin the lab by performing a serial dilution of either food coloring or lemon juice as instructed by your teacher. Exact measurements are critical for this lesson. Use the graduated cylinders carefully!! Be sure to rinse the cylinders between uses. Continue your dilution until you have reached 0.00001 percent solution or 1 part per million (ppm)
6. When you are ready, exchange lab stations with the across your table. Select one volunteer from your group to test the containers using the methods you devised in step one.
7. Examine the containers and decide which one has the smallest detectable concentration of the solution. Write down the order you think the tubes belong in from most to least concentrated.
8. Check the pH of your liquids using the sensors provided by your teacher, ( pH probes)
9. Exchange answers with your partner team. How did you do?

 

 

data table for mixing solutions...

dilution	1/10	1/100	1/1000	1/10,000	1/100,000	1/1,000,000
secret code lemon juice	.	.	.	.	.	.
food coloring	.	.	.	.	.	.

data table for examining another team's solutions...

dilution	1/10	1/100	1/1000	1/10,000	1/100,000	1/1,000,000
secret code lemon juice	.	.	.	.	.	.
actual pH measured with sensor	.	.	.	.	.	.
food coloring	.	.	.	.	.	.

discussion questions:

what methods did you use for detection?

 

 

why did you succeed/ fail to determine the dilution of the other teams mixtures?

 

 

what other methods of detection would you like to try?

 

 

 which was harder to detect changes in concentration of pH or food coloring? why?

 

 

Part B: Understanding toxicity

When working with databases that involve chemicals it is important to understand not just the quantities of the materials but also the toxicity. We have discussed the concept of biomagnification. This exercise should help you to understand what creates the difference between a harmless and a lethal dose of the same material.

The measurement toxicologists have created to convey information about toxicity is called LD50 or lethal dose for 50 percent of the population. It is important when looking at these numbers to look at the following criteria.

what toxic chemical is being tested? what species of animal is being used in the test? how was the toxic chemical administered? what was the animals body weight in grams in relationship to the quantity of chemical being administered? of the total group of animals being tested what percentage succumbed to the substance?

Follow the links to look at these web pages on the subject and then list three substances and their LD50. Research some of the chemicals we have discussed in class such as oil, PCB's and DDT. You may also refer to silent spring and list some of the chemicals mentioned there.

1.What is a Material safety data sheet?
read here to answer this question - university of oregon chemistry teaching laboratories

 

 

2.What is LD50?
read here to answer the question. - - royal society of chemistry

 

 

3.What are the LD50 of some common chemicals?
read here to answer the question. - university of oregon chemistry teaching laboratories

 

 

 

4. How big / small is one part per million? per billion? describe in your own words