__________________
(Name)
HONORS BIOLOGY I
pH Investigation With Household Liquids and Organic Molecules
Purpose: To study the effect of acids and bases on organic molecules.
New Purpose:
__________________________________________________________________
__________________________________________________________________
Problem: What effect will ammonia and lemon juice have on milk’s ability to be functional?
Hypothesis:
__________________________________________________________________
__________________________________________________________________
Materials:
12 pieces wide range pH paper
8 pieces narrow range pH paper
ammonia
1 stirring rod
1 baby food jar
lemon juice
paper towels
dropping bottles
milk
Procedure:
1.
Line up 4 squares of wide range pH paper about 1 cm apart on a paper towel.
2.
Put one drop of water on the first piece of pH paper. Determine the pH and record.
3.
Put one drop of lemon juice on the second square of pH paper. Determine the pH and
record.
4.
Put one drop of milk on the third square of pH paper. Determine the pH and record.
5.
Put one drop of ammonia on the fourth square of pH paper. Determine the pH and
record. (Do last due to fumes.)
6.
Compare the results of the four substances you tested.
(a) Which is the most basic?
(b) Which is the most acidic?
(c) Is one of these substances neither acidic nor basic?
7.
___________________
___________________
___________________
Molecules that make up or are produced by living organisms usually can function only
within a very narrow pH range. To show how a biological molecule is affected by pH,
place 100 drops of milk in a baby food jar. Measure the pH of the milk by transferring a
drop from the stirring rod to a square of narrow range pH paper and also wide range pH
paper. Record each pH.
(d) Why should you measure the pH of milk with narrow range pH paper?
________________________________________________________________
2
8.
Add a drop of lemon juice to the milk in the baby food jar. Stir thoroughly. Measure the
pH of the solution with new pieces of narrow and wide range pH papers.
9.
Counting the drops, and stirring each drop, add lemon juice to the milk until you notice
an obvious change in the appearance of the milk. Measure and record pH using both
narrow and wide range pH paper. The milk changes appearance because the acid in the
lemon juice has changed the protein in the milk, causing the proteins to precipitate out of
solution.
(e) What evidence do you see of this? ________________________________________
10. Repeat steps 7-9 except use ammonia in place of the lemon juice. Counting drops and
stirring after each drop, add ammonia to the milk until the difference in the pH of the
milk is equal to the difference in pH measured in step 9 for lemon juice.
(f) Does the appearance of the milk change after this point?
__________________________________________________
Summary/Conclusion – These answers should be in complete sentences.
1. Was the difference in the pH of the milk when you stopped adding lemon juice the same as
the difference when you stopped adding ammonia? Why is this important?
______________________________________________________________
______________________________________________________________
2. Why did the lemon juice curdle the milk but the ammonia did not?
______________________________________________________________
______________________________________________________________
Table 1: Wide and Narrow pH for Water, Lemon Juice, Milk and Ammonia
Substances Tested
Water
Lemon Juice
Milk
Ammonia
Wide Range pH (record number)
Narrow Range pH
100 drops milk
100 drops milk + 1 drop lemon juice
100 drops milk + lemon juice until obvious change
100 drops milk (2nd test)
100 drops milk + 1 drop ammonia
100 drops milk + ammonia (pH change equal)
Wide Range pH
3
Table 2: Number of Drops Needed for pH Change in Milk
Substance
Lemon Juice
Ammonia
# of Drops Needed for Obvious Change