USING ACID BASE INDICATORS TO TEST FOR UNKOWN SUBSTANCES
Background
Many items found at home are acids and bases. Many foods and cleaning products contain acids and
bases. Indicators are chemicals that can show whether a substance is acidic or basic. Two examples of
indicators are litmus paper and pH paper. Blue litmus paper turns red in an acid and stays blue in a
base. Red litmus paper stays red in an acid and turns blue in a base. PH paper will turn a different color
depending on the pH of the substance.
Problem
How can you determine whether a substance is an acid or a base?
Materials (per group)
5 drops of each test sample
ceramic sample dish
glass stirring rod
vials of red and blue litmus paper and pH paper
beaker of distilled water
1 10 ml graduated cylinder
NaOH solution
Buffer solution
Bromothymol blue
0.1N HCl
PART A
Procedure
Predict which substances will be acids, bases or neutral
1. Place 5 drops of each sample into a well of the ceramic dish.
2. Dip the glass stirring rod into the test tube and then touch it to a piece of red litmus paper. Do the
same thing to the blue litmus paper and to the pH paper.
3. Record your observations in the data table.
4. Rinse the glass stirring rod with water and dry it.
5. Repeat steps 2-5 for the other substances to be tested.
PREDICTIONS (10 PTS)
SUBSTANCE
PREDICTION
OBSERVATIONS (10 PTS)
SUBSTANCE
COLOR OF INDICATOR
RED LITMUS
APPROXIMATE
pH OF
SUBSTANCE
AMOUNT OF H+
IONS IN
SUBSTANCE
BLUE LITMUS
PART B
Procedure
1. Add 2.5 ml of NaOH solution to a test tube.
2. Add 1 drop of bromothymol blue and swirl until it is completely mixed.
3. Dip the glass rod into the solution and touch it to the pH paper. Determine the starting pH of the
dilute base solution. Record your observations in the table.
4. SLOWLY add 0.1 N HCl drop-by-drop into the flask. Keep count of the drops. BE CAREFUL
THAT THE DROPS ENTER INTO THE SOLUTION AND DO NOT ADHERE TO THE SIDES
OF THE TEST TUBE!! Swirl after each drop is added. Stop after there is a color change that
doesn’t disappear after 30 seconds. Record the number of drops in the data table. Record the new
pH.
5. Repeat steps 1-4 for the buffer solution.
Observations (10 pts)
SUBSTANCE
INITIAL pH
AMOUNT OF HCl
FINAL pH
ADDED
Conclusions
Part A
List your original predictions
1. Summarize and explain the results in part A. Include in your answer whether your predictions were
correct or not. (20 pts)
2. Were there any samples in which the litmus test and pH test were inconsistent? If so, explain why
you think this was so. (10 pts)
3. Is litmus paper useful in determining the exact pH of a solution? Explain your answer. (10 pts)
4. Suppose you are manufacturing a certain type of cosmetic. You know that it can be slightly acidic
but not strongly acidic. How would you go about determining how acidic your cosmetic is?
Explain your answer. (10 pts)
Part B
1. Summarize the results you obtained for part B. Explain the reason there were differences between
the NaOH and the buffer. (20 pts)