Honors Biology
Lab Investigating pH of Various Household Products
In an aqueous solution (any solute in a water solvent), a very small percentage of water molecules break apart
into ions. The ions formed are H+ (hydrogen ions) and OH- (hydoxide ions). A correct balance of hydrogen
ions and hydroxide ions is necessary for chemical processes to work correctly within a cell. A solution would be
called neutral when the amount of H+ and OH- is the same. A solution is considered acidic when the number of
hydrogen ions is greater than the number of hydroxide ions. Conversely, a solution is considered basic when the
number of hydrogen ions is less than the number of hydroxide ions. An acid is a molecule that donates H+ to a
solution, making it acidic. Hydrochloric acid (HCL) is an example of an acid. It disassociated into a hydrogen
ion and a chloride ion. A base is a molecule that adds hydroxide ions to a solution, making it basic. Sodium
Hydroxide (NaOH) is an example of a base. It dissociates into a sodium ion and a hydroxide ion.
The pH of substances can be found using a variety of tools and indicators within a laboratory setting. One of the
simplest tools, litmus paper, actually exists in three forms; red, blue, and wide-range. Acids will turn blue
litmus paper red and bases will turn red litmus paper blue. Wide-range pH paper, in comparison,
determines the exact numerical pH reading of each substance. In this lab you will test various household
products of your choosing to see whether they are primarily acidic or basic. You will then use wide range litmus
paper to get their exact reading. Finally, you will use wide-range litmus paper to observe what happens to a
solution of NaOH when you add drops of HCl to it.
Pre-lab Procedure (Must be completed before or on the day of lab)
In the lab section of your lab notebook, write the title and date of this lab. Then do the following:
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2.
3.
4.
5.
6.
Write the purpose of the lab.
Define “acid” and base” using the terms hydrogen ion and hydroxide ion.
Research and describe a neutralization reaction.
Research the chemical phenolphthalein. Write a statement about its use as it relates to acids and bases.
Read the Procedure A of the lab and create a data table for your results.
Read the Procedure B of the lab and create a section for your data.
pH Testing Procedure A
A. Litmus Paper Tests
1. Within the data chart, indicate the 5 substances that you and your partner will test.
2. Label 5 test tubes, one for each substance being tested. Pour about 5ml of each into the correct test tube. You
can estimate the 5ml using the measured 5ml in the sample test tube (don’t waste time measuring).
3. Collect 5 strips of blue litmus paper, 5 strips of red litmus paper and 5 strips of wide-range pH paper.
4. Back at your lab bench, make an educated guess as to whether each substance is acidic, basic, or neutral.
Mark your guesses in the data chart.
5. Dip the stirring rod into the sample test tube and add the substance to blue litmus paper. Dip the stirring rod
into the substance and add it to red litmus paper. Mark your results (acid or base?) in the data table. Rememberacids turn blue litmus paper red and bases turn red litmus paper blue. You need to use BOTH papers to figure
out if a substance is acid or base.
6. Dip the stirring rod again into the substance and add it to the wide-range paper. Use the scale on the side of
the dispenser to check your results. Mark your results in the data chart.
7. Repeat steps 5 and 6 for substance 2-5.
Neutralization Reaction Procedure B
1. Add 80 mL of water to a clean 100ml beaker. Determine the pH of water. Record the pH.
2. Using a 1ml pipette, add 3 drops of phenolphthalein to the water. Stir the water using a stirring rod.
Test and record the pH again. Wash the stirring rod.
3. One by one, add 4 drops of ammonia. Stir the solution. Note the appearance of the water as the ammonia
is added.
4. Record what happens to the water when the ammonia is added to the phenolphthalein and water. Test
and record the pH of the solution.
5. To the same solution, add one by one some drops of vinegar. Stir after each drop of vinegar is added.
Wait and then add another drop until the solution becomes clear.
6. Determine the pH of the clear and colorless solution.
Analysis: (To be answered in complete sentences in the lab section of the notebook).
1. What is the possible range of pHs for all the solutions tested?
2. Which of the five substances you tested were acidic? How did you know using the Litmus paper?
3. Which of five substances you tested were basic? How did you know using the Litmus paper?
4. Were any substances neutral? How do you know?
5. Did the results using litmus pH paper agree with the results using the wide-range paper? Explain.
6. What additional information did the wide-range pH paper provide?
7. Compare the pH value and the color of the ammonia solution with phenolphthalein. What do these values
tell about the solution?
8. Compare the beginning initial pH value of the solution in Procedure B with the final pH value.
9. The objective of Procedure B was to demonstrate a neutralization reaction which produces a salt and water.
Do the results you obtained in the neutralization investigation support the objective? Explain.
Conclusion: