9 Serial Dilutions of Acids and Bases

Transcription

1 9 Serial Dilutions of Acids and Bases Investigating Acid and Base Dilutions In Activity 8 you observed that there are indicators that can tell you whether a substance is an acid or base. You also learned that some indicators are useful over a wide range, while others can only be used over a more narrow range. For example, phenolphthalein can only be used to detect bases; it cannot distinguish between neutral substances and acids. In contrast, universal indicator and ph paper are wide-range indicators; they provide information throughout the range from extreme acid to extreme base. You will now examine how you can use a standard scale to estimate the ph number of a solution. Vinegar and many photographic chemicals are weak acids. A-57

2 Activity 9 Serial Dilutions of Acids and Bases CHALLENGE Set up a serial dilution to see how indicators behave as the concentration of acid and base changes. Look for patterns in the behavior of serial dilutions of acid and base when they interact with universal indicator. MATERIALS For each group of four students: 1 30-mL dropper bottle of each of the following: distilled water universal indicator solution 1 ph color chart 1 30-mL dropper bottle of either: 1% hydrochloric acid (HCl) or 1% sodium hydroxide (NaOH) solution For each pair of students: 1 SEPUP tray 1 stir stick 1 dropper 7 pieces (half strips) of ph paper 1 piece of white paper 1 paper towel SAFETY NOTE: Do not taste or touch the solutions. Use safety eyewear. Wash your hands after completing the activity. A-58

3 Serial Dilutions of Acids and Bases Activity 9 PROCEDURE 1. Set up your investigation report and data table so they are similar to the example shown below. 2. Put 10 drops of water in Cup 7, on the lower level of your SEPUP tray. 3. Perform a serial 1/10th dilution of the solution you are testing (A or B) in Cups 1 6. (If necessary, refer to Activity 3 to remind yourself about the dilution procedure.) Put water in one of the large cups for rinsing the dropper and stir stick between dilutions. 4. Test the ph of each solution with ph paper. Record the results. 5. Test the ph of each solution with universal indicator. Record the results. Dilution of Solution Color and ph Cup Dilution ph paper Universal Indicator 1 1/ / A-59

4 Activity 9 Serial Dilutions of Acids and Bases 6. Arrange the tray of acid and the tray of base so that the small cups are arranged, left to right, from most acid to most base, with the neutral cups in the middle. To do this, you will have to place the trays end to end. You may wish to have them overlap where they are neutral. 7. Complete the Analysis section. ANALYSIS Refer to the results of your group s investigation and the information in the following reading, Acids, Bases, and ph to answer the following questions. 1. Draw a diagram of your trays that you arranged from most acid (left side) to most base (right side). a. Label the cup that contains the known standard with ph 7, which is neutral. b. Draw an arrow that points from ph7 to the strong acid. c. Draw an arrow that points from ph7 to the strong base. d. Label each cup with the color and approximate ph of the solution. 2. How does the color change in your serial dilution relate to ph? Write a brief description that you could use to teach someone about this. A-60

5 Serial Dilutions of Acids and Bases Activity 9 Acids, Bases, and ph The ph of a solution is a value that expresses the solution s acidic or basic nature. A ph value may fall anywhere on a scale from zero (strong acid) to 14 (strong base). A value of 7 represents neutrality. Strongly acidic Neutral Strongly basic Bacteria Plants (algae, rooted) Carp, catfish, some insects Bass, bluegill Snails, clams, Largest variety of animals (trout, mayfly nymphs, stonefly nymphs, caddisfly larvae) Many aquatic plants and animals can live only within certain ph limits. A-61

6 Activity 9 Serial Dilutions of Acids and Bases CHALLENGE Relate the information in the reading to what you learned about ph in your serial dilutions of acid and base. PH AND ITS MEASUREMENT The measurement and control of ph is important in the manufacture of foods, paper products, and chemicals. In agriculture, it is necessary to maintain the correct soil ph for good yields of crops such as wheat, barley, corn, and other fruits and vegetables. Scientists also measure ph when they study acid rain and when they work to maintain water quality. The federal government s standard for drinking water requires the water to have a ph between 6.5 and 8.5. If drinking water is too acidic or basic, it may be harmful to human health. It also may react chemically with the water pipes (which are usually made of metal) to produce byproducts that contaminate the drinking water. Many aquatic animals and plants cannot survive in ph levels below 6 or above 8.5. Using indicators substances that change color with a change in ph is a way to measure ph. The color changes are the result of chemical reactions between the indicator and the acid or base. Acids turn ph paper red. Bases turn ph paper blue or violet. Acids and bases, and the salts produced by chemical reactions between them, are major groups of chemical substances. Originally, acids were recognized by their sour taste and because they could attack and dissolve some metals. Base solutions are usually slippery to the touch (think of bleach) and react with acids in water to form salts. Battery acid Lemon juice Vinegar Orange juice Boric acid Corn Milk Distilled water Blood Sea water Baking soda Borax Milk of magnesia Ammonia Bleach Household lye Strongly acidic Common range for most natural waters Strongly basic ph Scale A-62

7 Serial Dilutions of Acids and Bases Activity 9 Acids and bases have important uses in the manufacture of many products. For example, in 2000, more than 5 million short tons of sulfuric acid were used by industry to make other chemicals and chemical products (a short ton is 2,000 lbs.). It is used in making fertilizer, refining petroleum, and in the industrial cleaning and processing of metals. Nitric acid is used in manufacturing explosives and dyes. The bases sodium hydroxide and potassium hydroxide are used in soap-making. Ammonia, another base, is used as a fertilizer and in the production of other fertilizers. QUESTIONS 1. Why is the ph of water important to living things? 2. Explain how the dilutions in your trays can be used as a model for the ph scale shown on page A-62. A-63