Disease Transfer
Age
Elementary School, Middle School, High School
Format
Stage/Hands-on (requires a group of about 15-40 students)
Materials
Paper cups (one for each student) DI water (~30 mL per cup, needs to be neutral!) NaOH solution (0.3 molar) Phenolphthalein indicator solution & eyedropper
Safety Precautions
Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Make sure students do not try to drink from their cups, even dilute NaOH should not be swallowed! Also try to minimize expose to skin.
Preparation
Note that this demonstration will not work well for very small or very large groups. An audience of about 15-40 students should work well.
Prepare the NaOH and indicator solutions beforehand.
Pour ~30 mL of water into each cup, enough for every student to have one.
Demonstration
Give each student a cup with water. Randomly pick about one tenth (1/10) of all the students. Add about 5 mL of the NaOH solution to those selected students' cups.
Instruct all the students to each find three other students share their water with. To share the water, instruct them to first pour one cup of water into the other cup, then pour half that mixture back into the first cup.
Once every student has mixed their water with three other students, go around and add a few drops of the indicator to each cup. Compare the number of students that started out with a basic solution to the number of students that ended up with one.
What to Say
This demonstration can easily be adapted for younger and older audiences. Younger audiences may be more interested in the concept of basic hygiene - how hand washing and bathing can prevent them from getting sick. Older audiences may be interested in the spread of sexually transmitted diseases.
A sample script for young audiences:
I bet every single one of you hears your parents tell you to wash your hands every day. You all do it, too, right? That's good - do you know why they tell you to wash your hands?
That's right, because if you don't wash your hands, you can introduce harmful germs to your body and get sick. But how big of a threat are these germs really?
In fact, there is a very serious danger of catching a cold or another disease from bacteria or viruses that you might find on a doorhandle, a computer keyboard, your friend's hands, or even in the air you breathe. Washing your hands to kill these germs really does help keep you healthy.
(Now pick 1/10th of the audience - if it's a class of 30, pick 3 students)
Today we're going to see just how quickly a disease can spread. We're going to give you all a cup of water - this water represents your own body. Now, you three special volunteers are actually not so lucky - you're going to start off with a disease. Right now I'm adding a special chemical to your water that represents the virus in your body that is causing this disease.
Unfortunately, this chemical is clear and you can't see it in the water, so no one else knows you have the disease. That means, if you give the disease to your friend, someone who meets your friend will have no idea that they're coming into contact with the disease.
So, everyone in the class needs to find a partner and they are going to mix their water with that partner. Just pour both of your volumes of water together into one cup, and then pour half that water back into the other cup. Then I want you to find another, different person to do the same thing with, and then another. In total, you need to share your water with three different people!
While you guys are mixing your water, think about what this activity represents. Sharing your water is like bringing your body into contact with this other person - this could mean shaking hands with them, touching something after they have touched it, or even just breathing the same air they are breathing.
Ok, now I am a doctor. I am going to go around and test you all for the disease. I'm going to add a chemical to your water that will turn pink if you have the disease.
At first, only three people had that disease. Do you think you have the disease now? Did you come into contact with one of those three people? How many of your classmates do you think have the disease? Do you think it's twice as many people? Do you think it's three times as many people?
Ok, let's count. Wow, X many people have the disease now! Isn't it amazing quickly it spread? Think of how many people you interact with during the school day - if one person brings a germ to school, it could easily find its way into nearly anyone else in your school. But, you can stop those germs! Washing your hands is a great way to kill those germs and prevent the spread of diseases such as the flu or colds.
You should also offer a very brief explanation of the acid/base chemistry that is helping to illustrate this demonstration (see below).
Why It Is
This is really a demonstration of exponential growth which we are applying to one real-life example, the spread of diseases. Every time you have the audience mix their water, a disease-carrier turns into two disease carriers. In other words,
[Disease carriers] = [Initial disease carriers] x 2
So how many students would you expect to catch the disease by the end of the demonstration?
[Disease carriers] = [Initial disease carriers] x 2 x 2 x 2
Eight times as many students as you started with may end up with the disease, although slightly fewer will probably catch it because sometimes two disease-carriers will mix water with each other, in which case no one new gets the disease.
In general,
[% disease carriers] = [Disease carriers] / [Total population] = [Initial disease carriers] / [Total population] * 2^N = 2^N / 10
where N is the number of times you mix the water. The factor of 2 ^ N is where the phrase "exponential growth" comes from.
The phenolphthalein indicator is just an acid/base indicator. Water is neutral, so the phenolphthalein does not change color. Students that carry some NaOH in their water will have a basic (high pH) solution, so the indicator will turn pink.
Real Life Examples
The spread of disease and other exponential growth processes, including population growth and investment interest.