Tuesday, October 14, 2008

T.E.S.L.A. Lesson Plan: pHerfect!

I thought it might be a good idea to start posting my lesson plans for T.E.S.L.A. on my blog. Maybe there is a homeschooling parent (or a teacher who doesn't wet her pants just thinking about administrators lecturing her about liability insurance) who would like to have them.

Today's lesson is called "pHerfect!" and it is about pH. The T.E.S.L.A. format I've come up with goes as follows. There are always two very brief lessons, each of which is followed by at least one experiment, and an activity that generally results in something to take home. It lasts about an hour.

Objective: to make students aware of pH, testing for pH, and natural pH indicators.

Materials: various household substances (see list under Experiment 1)
100 pH testing strips or litmus paper
poster of color key that came with the testing strips
1 head of red cabbage
baking soda
coffee filters
handwash station (if indoors, this can be a sink)

Advance Prep: puree half a large head (or 1 small head) of red cabbage. Cover with boiling water, steep for 10 minutes. Strain and press out as much liquid as possible. Soak coffee filters in liquid for 2 hours. Remove coffee filters; reserve liquid. Hang soaked filters to dry.

Lesson 1: What is pH? pH is a measure of how acidic or basic things are. How do we test pH? We use this special paper called litmus paper. It changes color according to the pH of the substance. Here is the color chart poster showing what color this paper will turn. This is how we test things [demonstrate, lay out rules for testing: divide into groups, no pushing, take turns, do not remove substance from table, if you get some on your hands please wash at handwash station, etc.]

Experiment 1: pH testing. Students will record the pH of each substance they test on a piece of paper located at each station. Substances can be tested more than once by different students. The results will be reviewed at the end of the experiment.

Station 1: water
distilled water
tap water
bottled water
spit (self-provided)

Station 2: solutions
baking soda solution
apple juice

Station 3: fruit/veg

Station 4: apple varieties
Red Delicious
Golden Delicious
Granny Smith

Station 5: household chemicals
soapy water
phosphoric acid

Lesson 2: anthocyanins I made this purple liquid from red cabbage. Red cabbage, along with many fruits and vegetables, gets its color from molecules called anthocyanins. These molecules change color when mixed with acids and bases. [Demonstration: two identical beakers or glasses with some diluted cabbage infusion. Add vinegar to one and baking soda solution to the other. Vinegar will turn it red and baking soda will turn it blue.] This is why hydrangeas change color according to whether their soil is acidic or basic.

Experiment 2: mixing acids and bases with infusion of red cabbage. Students (with adult help as appropriate) will mix some of the liquids from Experiment 1 with dilute red cabbage infusion and compare the results with the previous pH measurements and with the other red cabbage results. Students observe that more acidic substances turn redder and more basic substances turn bluer.

Take-Home: coffee filter soaked in cabbage infusion and dried (homemade litmus). Extra time can be filled having students cut the filter in strips. Students should be encouraged to come up with their own experiments to do with it at home, here are some suggested ones:
  • measure the pH of your spit, then chew gum and measure it again. Later on, brush your teeth and measure the pH. Measure the pH of your spit immediately upon waking.
  • compare the pH of various fruit juices. Is there a relationship between pH and how "tangy" a juice is?
  • measure the pH of your urine (get parental permission first)
  • measure the pH of your bath water with and without bubble bath, or before and after bathing
  • see if adding sugar, salt, baking powder, or other substances to water changes its pH