pH and Living Systems
Scientists use something called the pH scale to measure how acidic or basic a liquid is. The scale goes from 0 to 14. Distilled water is neutral and has a pH of 7. Acids are found between 0 and 7. Bases are from 7 to 14. Most of the liquids you find every day have a pH near 7. They are either a little below or a little above that mark. When you start looking at the pH of chemicals, the numbers go to the extremes. Substances with the highest pH (strong bases) and the lowest pH (strong acids) are very dangerous chemicals. Molecules that make up or are produced by living organisms usually only function within a narrow pH range (near neutral) and a narrow temperature range (body temperature). Many biological solutions, such as blood, have a pH near neutral.
The biological molecule used in this lab is a protein found in milk. Proteins are used to build cells and do most of the cell's work. They also act as enzymes. For proteins to work, they must maintain their globular shape. Changing the shape of a protein denatures and the protein will no longer work.
Small squares of wide-range pH paper, pH color chart, paper towels, 4 dropper bottles, ammonia, lemon juice, skim milk, distilled water, forceps, 50 ml beakers, small squares of narrow-range pH paper, 2 stirring rods
Procedure (part A): Testing the pH of Substances
Questions (Part A): Determining the pH of Solutions
Procedure (part B): Showing the Effect of pH on a Biological Molecule (Milk Proteins)
|Substance Tested||Substance used to Produce Change||Starting pH of Milk||Final pH of Milk||Original Appearance of Milk||Final Appearance of Milk||Total Number of drops added to Produce the change|
|100 drops Skim Milk||Lemon Juice|
|100 drops Skim Milk||Ammonia|
1. Which substance tested from table 1 was the most acidic?
2. Which substance was most basic?
3. Did any substance from table 1 have a neutral, or near neutral pH? If so, which substance was neutral?
4. Why did you use narrow-range pH paper to measure the milk's change in pH?
5. Describe the change in appearance of the milk as more lemon juice was added. Explain why this change occurred.
6. How much did the pH of milk change when lemon juice was added?
7. Why do you think lemon juice "curdled" (precipitated out the proteins) from the milk?
8. Did you get the same change when ammonia was used? Why or why not?