Acids and bases are common substances in nature and in consumer products. In chemistry, acids are substances that provide hydrogen ions, with the symbol H+. If a water solution has lots of H+, it is very acidic, and if it has very little, it is basic. In the middle, we call it neutral. Hence the acidity or basicity is determined by the H+ concentration in a solution. Pure water has a pH of 7. This H+ concentration can be expressed mathematically in a scale, called the pH scale. pH 7 is considered neutral; pH lower than 7 is acidic and higher than 7 basic. One way to tell if a substance is an acid or a base is by its taste. An acid is sour, and a base is bitter. Obviously it is not smart to taste everything you encounter. Luckily there are many compounds that can serve as acid-base indicators. One of those is phenolphthalein used in Demo 15. These indicator compounds change color at various concentrations of H+, and are therefore made used of to determine the pH of the solutions to which they are added. The reason they change color is that their molecular structure is affected by the H+ in such a way that they absorb light at a different wavelength. The following demonstrations will illustrate the colorful behavior of acid-base indicators.
Demo 16 Caught Red Handed
Materials: galaxy-golden paper, washing soda, lukewarm water, glass bowl Precautions: Wear goggles. Washing soda is very basic; wash your hands thoroughly after performing this demonstration.  Disposal: The paper can go into the trash. The washing soda solution can be flushed down the drain. Procedure: Stir one tbsp of washing soda into one quart of lukewarm water in a large bowl. Some solids may stay in the bottom of the bowl. Soak your hand in the bowl of washing soda solution. Take your wet hand out and slap your palm on a piece of galaxy-golden paper to make a “bloody” palm print. Wash your hand. Explanation: The pigment used to make the golden-yellow color of the paper changes in molecular structure after coming in contact with the basic washing soda solution. This change of structure causes it to absorb light at a different wavelength and to make it look red. Turmeric is a spice that gives food an intense golden-yellow color. It too will turn red when a base such as baking soda is mixed with it.
Demo 17 Is Red Cabbage Red
 Precautions: Wear goggles. Avoid getting any of the chemicals in your eyes. Read the product labels. Disposal: All solutions can be flushed down the drain. The pieces of pH paper can be put in the trash. | Procedure: Boil two to three red cabbage leaves in two cups of tap water for five minutes giving a fairly dark purple liquid. (Microwave cooking will also work.) Dilute the cabbage juice with three cups of tap water in a pitcher. (If the color is too dark, the changes are not as vivid.) Set up six clear plastic cups. The first cup contains one tsp Lime-Away, the second cup one tsp vinegar, third cup water (or leave empty), fourth cup 1/4 tsp baking soda, fifth cup one tsp ammonia, sixth cup 1/8 tsp lye. Pour some diluted cabbage juice into each cup to about one third full. Swirl the cups to mix the contents. Different colors appear. If further investigation is desired, calibrate the colors with standard pH paper by dipping pieces of pH paper into each solution and compare the color of the pH paper to the colors on the chart that comes in the box of pH paper.
 Explanation: . The red cabbage (Brassica oleracea) contains pigments known as anthocyanins. Anthocyanins belong to a large group of water-soluble natural pigments responsible for the attractive colors ranging from strawberry red to the blue color of most fruits, flowers, leaves, and some vegetables. Anthocyanins are commercially used as a colorant in acid solutions such as soft drinks. At various H+ concentrations these compounds rearrange their molecular structures giving rise to different colors. You can compare these colors to colors of standard pH paper. Doing so will give you a standardized pH indicator solution that you can use to determine the pH of an acidic or basic solution of interest. Demo 18 Red Tornado
Materials: Alka-Seltzer®, Zep® (containing hydrochloric acid) (can use any one of the other acids, such as citric acid, sulfamic acid, sulfuric acid.), red cabbage juice (see Demo17 Procedure for preparation), tall clear glass cylinder, spoon.
 Disposal: Flush solutions down the drain. Procedure: Put the red cabbage juice into the glass cylinder. Adjust the color of the solution to red by adding a few drops of Zep slowly. When you are ready to perform the demonstration, use the spoon to stir the liquid in a circular motion to create a vortex. Open a foil packet of Alka-Seltzer and drop one tablet into the cylinder of red cabbage juice and watch the effervescent effect. Explanation: This is an acid-base neutralization reaction. The Alka-Seltzer is more basic than the red cabbage juice and thus neutralizes the acid that makes the red cabbage juice red. Consequently, the anthocyanin pigments in the red cabbage juice change colors. From the colors obtained in Demo 17 can you tell what pH this solution is after the neutralizing action Alka-Seltzer? Alka-Seltzer is one of the many antacids on the market. The sodium bicarbonate from the tablet neutralizes the hydrochloric acid in the stomach. Other well known antacids are Tums® (calcium carbonate), Rolaids® (aluminum sodium dihydroxycarbonate), and Phillips® milk of magnesia (magnesium hydroxide). |
Acid-Base |