Identification of an Acid Or Base

Identification of an Acid or Base Lab Name______Date______Background Information: Acids and bases are unique substances that are all around us and critical to living systems. Many of the items that we use daily can be classified as either an acid or a base. Acids have a characteristic sour taste and bases have a bitter taste; however, tasting such substances is never an option in the lab. The identification of acids and bases most commonly involves determining the [H+] concentration of the solution.

Acids are substances that when dissolved in water increases the [H+] concentration. Bases are substances that when dissolved in water increases the [OH-] concentration, therefore decreasing the [H+] concentration.

The values for [H+] and [OH-] concentration are generally so small that scientists in the medical profession developed the pH scale, ranging from 0-14, to report the acidity or basicity of common substances. A pH value less than 7 is acidic. A pH value of 7 is neutral (neither acidic or basic). A pH value larger than 7 is basic.

In this activity, you will use two indicators pH of various solutions. Indicators are substances that change color depending on the pH of a solution. Blue litmus paper turns red in the presence of an acid. Red litmus turns blue in the presence of a base. Another type of indicator, pHydrion paper, changes multiple colors over a wider range of pH changes.

Procedure: Caution: DO NOT touch, taste or inhale any of the solutions used in this activity. Please wash hands upon completion of this activity.

1. Obtain red and blue litmus paper, a roll of pHydrion paper. 2. Make sure that the 14 small plastic cups, labeled 1-14, are arranged in order. 3. Familiarize yourself with the color key on the pHydrion paper dispenser. 4. Begin by testing all of the solutions with the red and blue litmus paper. Remember that red litmus will turn blue in a base and blue litmus will turn red in an acid. Record “y” for yes or “n” for no if the litmus paper changes color on Data Table 1 below. 5. Next, use the pHydrion paper. Record the color of the paper and the corresponding pH in Data Table 1 below. Use a new strip of paper for each solution. DO NOT waste the paper. Only short strips are necessary. 6. Dispose of the litmus paper and pHydrion strips in the trashcan. Return the cups to the instructor. Clean your lab area.

Questions:

1. Which solution (pH of 1 or pH of 6) has the higher [H+] concentration (and therefore is a stronger acid)?

2. Which base (pH of 8 or pH of 13) has the higher [H+] concentration (and therefore is a weaker base)?

3. List the items from the activity that would be used as household cleaning agents in order by increasing pH value. Were they mostly acids or bases? 4. List the items from the activity that would be consumed as food in order of increasing pH. Were they mostly acids or bases?

5. Normal digestion occurs in an acidic environment (gastric acid). Too much acid can cause indigestion (heartburn, acid reflux). Describe what effect milk of magnesia would have on your digestive system. Use the results from the lab to support your answer.

Data Table 1: Determination of pH Values for Various Substances Red Blue pHydrion Litmus Litmus paper Solution Prediction Acid or Base change change color & (y/n) (y/n) value 1. ammonia

2. tap H2O

3. tea

4. coke

5. vinegar

6. milk of magnesia

7. lemon juice

8. milk

9. juice

10. all purpose cleaner 11. window cleaner

12. dish detergent

13. unknown A

14. unknown B

15. unknown C