Brad I - 9th Grade Biology - Communicable Disease

Principals Investigated: Epidemiology, vectors, communicable disease

Duration: 1 hour
Standards: Physiology 10. a-f, Investigation and Experimentation 1.a, c, d, m.


2 plastic cups (or test tubes) per student.
Dilute sodium hydroxide solution (0.01 N; add 0.4g of NaOH to 1L H2O)
Phenolphthalein solution
Digital chart to record data (included in the power point presentation attached at the bottom of the page).

Prior knowledge:
Students know the various causes of disease
Students know how infectious diseases are transmitted
Students know the body's nonspecific defenses against invading pathogens
Students know the function of the immune system
Students know relevant vocabulary terms including: disease, pathogen, germ theory of disease, vector, immunity

Label cups as follows: Each student gets 2 cups labeled with the same identifying number.  One cup should also be marked with a "C" for control and the other marked with an "E" for experiment.  The numbering system should be sequential, i.e. The 9th student should have two cups, one labled 9C and one labeled 9E.  All cups should be filled about 1/3 of the way with water except one set (e.g. 9E and 9C).  The set not filled with water should be filled to the same level as the others, but using the dilute sodium hydroxide solution.  Make sure that you keep track of which set of cups is filled with the sodium hydroxide.

When beginning the lab explain to students that when they are prompted they should walk around the room and mix the contents of their cup with that of a classmate.  Students must also record the number of the person they mixed with.  Demonstrate mixing to students by pouring the contents of two cups back and forth a few times then splitting the resultant mix.  Next, pass out all of the cups to the students.  Again, each student gets a numbered "experiment" cup with the corresponding "control" cup.  Give the students about 1 minute per trial then have them return to their seats   This process should be repeated until you have 3-4 trials (3 trials for classes pf 20 or fewer, 4 trials for classes greater than 20).  Alternatively, instead of having them return to their seats each time, you can instruct the students to mix with 3 different individuals, recording the number after each trial.  Once students are back in their seats the instructor goes around the class and adds a drop or two of phenolphthalein to each experimental cup (i.e. not the control cups).  Those cups whose liquids turned pink represent infected individuals.

Next, fill in the digital chart by calling on students and asking them what numbers they mixed fluids with.  Give students time (and pointers, if needed) to try and deduce whom the original carrier was.   After they seem to have narrowed it down, ask the students what their logic was in solving this puzzle.  Also, ask them if they can narrow it down to exactly one individual (and why?).

Finally, to reveal the original source of the disease go around the room and add the same amount of phenolphthalein to each "control" cup.  Only the original source cup (sodium hydroxide) will turn pink.  For effect, since you know who the original source is, add phenolphthalein to their cup last.

Students will be able to narrow down the original carrier to two individuals.
Students can identify tubes that could not have been the source (i.e. if a test tube is clear after the final trial, then none of the tubes that mixed with it could be the source).

Real World Application:
This activity shows how quickly communicable disease can be spread.  It also emphasizes the virtue of disease prevention by any means including abstinence and vaccinations.
Norman Herr,
Apr 25, 2011, 5:39 PM