Lab: Acids and Bases Objectives • Understand the properties and reactions of acids and bases • Relate these properties to common household products Introduction Chances are you’ve seen the commercials talking about acid-reflux disorder. Acid-reflux is a condition that occurs when the pH of stomach acid has decreased and is damaging the lining of the stomach or esophagus. Many people rely on over-thecounter tablets or liquids to relieve the symptoms of acid-reflux and nausea. You may be more familiar with a simple nausea remedy - warm lemon-lime soda pop. Why do these substances work to relieve nausea and acid-reflux? These substances have higher pH levels than your stomach acid. Antacids are either a base or a basic salt and help to raise the pH of your stomach acid by neutralizing the excess acid. The same is true of soda pop, an acid with higher pH than your stomach acid. Figure 1: Antacid tablets and liquids are overthe-counter medications that people use to reduce the effect of a decrease in the pH level of stomach acids. Compounds, like acids and bases, are very important in nature and in chemistry. Chemist Svante Arrhenius defined acids and bases this way: acids dissociate in aqueous solutions to form hydrogen ions (H+), while bases form hydroxide ions (OH-). Johannes Brønsted and Martin Lowry concluded that acids donate hydrogen ions (H+) to bases (OH-), which in turn, receive the hydrogen ion (H+). Acids and bases are physically different in many ways. Acids and bases, which you can ingest, do not taste the same. An acid, like orange juice, tastes sour, and a base, like soap, tastes bitter. Bases, which are normally found as cleaning products in the house, also feel slippery. You should always exercise safety when working with any acid or © KC Distance Learning base. Both acids and bases can burn or dissolve human tissue. This works in your favor in many different ways. Have you ever swallowed one of your teeth? Your stomach acid has low pH and is capable of dissolving many different materials, including teeth and bones. So what is pH and how does it relate to acids and bases? As measured on the pH scale, pH refers to a standard set of acid and base solutions in concentrations that have been agreed upon by an international group of scientists. On this scale, pure water is neutral at 7.0, solutions with a pH of less than 7.0 are considered acidic, and those with a pH of greater than 7.0 are considered basic. Don’t confuse the pH scale with p[H3O+]. In chemistry, p[H3O+] is the measure of the molar concentration of the number of dissolved hydronium ions (H3O+) in a solution. The p[H3O+] scale is Figure 2: Sample pH scale showing the pH level of logarithmic, so 0.1 molar of hydrochloric various acid and base solutions. Drinking water, acid has an approximate pH of 1.0, while which can be affected by minerals and other dissolved solids, has a pH ranging from 6.5 to 7.5 on 0.001 molar of HCl as a 3.0 pH. There the scale. Drinking solutions, such as seawater, milk are many ways to measure pH, including of magnesia, or other bases causes your body to using pH indicator strips, electronic pH dehydrate and can be dangerous to your health. meters, and even a simple red cabbage juice test for measuring pH. Use the following web site to create the red cabbage juice pH indicator: http://science.howstuffworks.com/innovation/experiment1.htm Always remember that acids and bases are some of the most reactive and dangerous chemicals you may encounter. Individuals with bulimia or anorexia nervosa, conditions which include constant vomiting, can burn their esophagus and mouth as well as dissolve their teeth. Every day many individuals are rushed to emergency rooms with burns from exposure to battery acids, while small children can easily be burned by drinking basic solutions, such as drain cleaners. One of the most important safety practices in chemistry is the protocol used when mixing acids or bases with water. Any © KC Distance Learning time you are mixing an acid or a base with water, always pour the more concentrated solution (acid or base) into a larger volume of water. Pre-lab Questions 1. What type of acid is stomach acid most similar to on the pH scale? 2. What would happen when you mix hydrochloric acid (HCl) with sodium hydroxide (NaOH)? 3. Estimate the pH of the resulting solution if you mixed equal amounts of hydrochloric acid (HCl) and sodium hydroxide (NaOH). © KC Distance Learning Experiment: pH Scale In this experiment, you will be working with a simulated lab. Study the screen shot of the simulator and pay close attention to the highlighted controls. These controls, along with others specified in the lab procedures will assist you in successfully completing this experiment. As you conduct each portion of the experiment, be sure to write down your observations. NOTE: If your experiment gets out of control, you can start over by clicking the reset button located on the right control panel. Materials: PhET lab – pH Scale © KC Distance Learning Procedure 1. Start the lab with a beaker containing 1.00 liters of milk with a pH of 6.50 on the pH scale. 2. Observe the concentrations of the hydronium ions (H3O+), hydroxide ions (OH-), and water (H2O) in the “Water Components” chart on the right of the screen. 3. Switch the chart from a “Logarithmic” scale to a “Linear” scale using the radio buttons located below the chart. Observation: How did the chart change? 4. Using the “Zoom In (÷10)” button on the chart, zoom in eight times. As you zoom in, observe how the scale on the left-hand side of the chart changes. Observation: What is the relationship of the number of hydronium ions (H3O+), hydroxide ions (OH-), and water (H2O) in milk? 5. Use the reset button at the bottom of the screen to reset all the settings. 6. Use the drop-down menu in the upper-left corner of the screen to add 1.00 liters of battery acid to your beaker. 7. Use the drain bibcock to drain the battery acid to a level 0.10 liters in the beaker. NOTE: This is a difficult procedure and a level of approximately 0.10 liters is acceptable. Observation: What is the relationship of the number of hydronium ions (H3O+), hydroxide ions (OH-), and water (H2O) in the battery acid? What is the pH of the battery acid as indicated on the pH scale in the center of the screen? © KC Distance Learning 8. Observe the number concentration on the “Water Components” chart. 9. Use the water bibcock to add 0.10 liters of water to the solution to create a 0.20liter solution of battery acid. Observation: How did the pH level and the water components level change after adding water to the battery acid? 10. Use the water bibcock to add 0.30 liters of water to the solution to create a 0.50liter solution of battery acid. Observation: How did the pH level and the water components level change after adding water to the battery acid? 11. Use the water bibcock to add 0.50 liters of water to the solution to create a 1.00liter solution of battery acid. Observation: How did the pH level and the water components level change after adding water to the battery acid? © KC Distance Learning 12. Repeat steps 5 through 11 for another liquid. Be sure to write down all of your observations. Post-lab Questions 1. How much water is required to dilute an acid or a base? Explain your answer. 2. Compare the results from the observations you made with the battery acid to those of the acid or base you observed in step 12 of the experiment. 3. List two acids and two bases you come in contact with on a regular basis. Which of these is has a pH farthest away from pure distilled water? © KC Distance Learning
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