Chemistry is pHun

Michelle Jones                 Morgan Park High School
                               1744 W. Pryor Ave.
                               Chicago IL 60643
                               (312) 535-2550

Objectives:

The main objectives of this mini-teach are to familiarize uppergrade students 
with the concept of pH, to teach students how to use litmus and pH paper to 
determine pH and to make students aware of natural indicators. 

Materials Needed:

For a class the teacher should plan for stations with six samples (2 acidic, 2 
basic and 2 neutral) probably use groups of two or three.  !!Caution!!  Do not 
use bleach or bleach containing products.  Do not use products that don't list 
chemicals. 

Baking soda solution                  Table salt solution
Lemon juice                           Vinegar
Ammonia/Windex                        Distilled water
Clear containers (Baby food jars, cups)
Grape juice                           Fruit punch
Beet juice                            Red cabbage juice (in rubbing alcohol)
Litmus paper                          pH paper
Tweezers                              Paper towels

Strategy:

Activity One:  Measuring pH
1.  Have stations set up for students ahead of time.  Each station should 
include labeled cups or jars with six solutions (2 acidic, 2 basic, 2 neutral) 
grouped so no one person will test two neutral solutions.  The stations should 
also have tweezers for each student, instructions, paper towels for spills, and 
have small torn pieces of litmus paper and pH paper. 

2.  Vocabulary- INABP Tests
 Indicator- points something out, in this case tells if the solution is acidic, 
    basic, or neutral.
 Neutral-  1) not acidic or basic, 2) equally acidic and basic
 Acidic-   increases hydronium ion concentration, [H3O+], in water
 Basic-    increases hydroxide ion concentration, [OH-], in water
 pH-       1) scale of acidity,  2) pH= -log[H3O+]

3.  After explaining the vocabulary the teacher should explain that there are 
stations to test pH.  Then show students that by holding any indicator paper 
with tweezers and barely dipping it in a solution and then comparing the color 
with the provided chart, one can determine if the solution is acidic, neutral or 
basic.  With litmus paper it is easy to remember Acid turns Red (try to rhyme 
it) or remember that Bases are Blue (alliteratiion).  With pH paper a chart 
should be given and the color can be matched with the colors on the chart and 
the pH is estimated by the corresponding number (0-6 acidic, 7 neutral, 8-14 
basic).  

4.  Allow the students time to test the solutions at the stations and have them 
record results.  The class should come to agreement on the results.  Ask the 
student if there were any surprises.  The water may be slightly acidic because 
carbon dioxide in the air may dissolve in the water making carbonic acid, the 
same acid in soda pop.

Activity Two:  Calibration of Natural Indicators
1.  Explain to the students that there are other indicators than litmus and pH 
paper.  Provide them with the examples of the electronic pH meter and juices 
listed in materials.  (HINT add ammonia or another base to the juices first to 
get a wider range of changes and only have one juice at each station). 
 
2.  Instruct the student groups to add drops of vinegar to the juice to increase 
the acidity and watch for color changes.  At each new color the groups should 
measure the pH with pH paper.   

3.  Have the students return to stations, calibrate the juices, and share 
results on the board.

4.  Clean up.

Activity Three:  Drawing Ions
1.  Most students will know the formula for water is H2O.  This is demonstrated 
by drawing the atomic model which looks just like a Mickey Mouse Head.  The 
large head being the large oxygen atom with a charge of negative two, and the 
two smaller ears are the two smaller hydrogen atoms each with a charge of 
positive one, therefore, making the total charge zero. 
 
2.  For acids hydronium ions are made.  The model for this is a Mickey Mouse 
with an extra ear (a water with an extra H with a positive one charge making the 
total charge of the ion positive one). 

3.  For bases hydroxide ions are made.  The model for this is a Mickey Mouse 
with only one ear (one less hydrogen with a positive charge, making the total 
charge of the ion negative one).  
  
Performance Assessment:
The activities are for student exposure and I would hope they get a clear idea 
of what pH is and how it is measured.  I would have the students turn in 
observations sheets that record the pH of each solution and the color change and 
pH of each juice. 

Conclusions:
The teacher should give the student a quick review of the vocabulary and how 
they relate to solutions used almost every day. 
 
Material              pH         Material                pH 
Battery Acid          0          Blood                   7.2
Stomach Acid          1          Sea Water               7.8
Lemon Juice           2          Baking Soda             8
Vinegar               2.3        Borax                   9
Wine                  3.8        Milk of Magnesia        10
Tomatoes              4          Ammonia                 12
Black Coffee          5          Bleach                  12.3
Milk                  6.8        Oven Cleaner            14
Pure Distilled Water  7