Acid-Base Chemistry

Chemical compounds are grouped together based on similar properties. Acids are chemical compounds that are sour or bitter. These compounds when placed in water dissociate to form hydrogen ions. Bases are a group of compounds that have opposite properties to acids. Bases are characteristically slippery. A number scale called a pH scale, which spans from 0 to 14, is used to provide information on how strong an acid or base is. Values on the pH scale between and 7 are acidic; values above 7 are basic. A pH value of 7 corresponds to the neutral condition, neither acidic nor basic. Pure water is neutral. Food is often acidic in nature and household cleansers are often basic. A universal indicator will be used in this experiment to determine whether solutions of interest are acidic, basic or neutral. An indicator is a compound that when dissolved in water changes color depending on the acidic, basic or neutral condition of the solution.


Materials

 A universal indicator (pH = 1-13)

Alternatively red cabbage solutions can be used as indicators.

Test solutions (lemon juice, vinegar, tomato juice, milk, water from different sources, antacid, milk of magnesia, household ammonia, toothpaste, aspirin, vitamin C, scouring powder, detergent, shampoo and conditioner)

 Buffer solutions at pH = 2,4,6,8,10

Small glass vials (10 mL) with caps

 

What To Do

 Fill a glass vial almost all the way with a test solution. Add two drops of universal indicator and cap the vial. Shake the closed vial and observe the resulting color. Compare the color of the test solution to the colors of the buffer solutions. The pH of the test solution can be estimated by matching the color of the test solution to the color of the buffer solutions with known pH values. Find the pH value of a variety of test solutions by repeating the steps above. After the pH values of the test solutions have been determined sort the solutions into acid, base, and neutral groups.

 

Questions

1. Report the pH values of the solutions of interest or alternatively report whether the solutions are acidic or basic based on the observed color of the test solutions.

2. What trends do you notice among the food samples and among the cleansers?

3. Compare the pH values of hair shampoos and conditioners. What are the observed similarities or differences?

 

Summary

The universal indicator will have a bright red color for highly acidic conditions and will change to orange as the solution conditions approach neutral pH. More and more basic solutions will become blue in color. Test solutions can be grouped into acid, base, or neutral groups. The work can be checked by referring to the list of pH values for common household products.

 

Source

"Chemical Demonstrations: A Handbook for Teachers of Chemistry." Vol. 3., Bassam Shakhashiri, University of Wisconsin Press, 1989.

 "Science Is." Susan V. Bosak, Scholastic Press, 1991.

© S. Olesik, WOW Project, Ohio State University, 2002.