ONS10_se_Ch06_sec6_2_page229to235.pdf - Key Terms pH scale...

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Key Terms pH scale pH indicator 6.2 The pH Scale and Indicators In May 2001, 49 people who attended a dance festival in Dauphin, Manitoba, became sick within a week. The evidence suggests that the source of the illness was the hotel pool. In Chapter 5, the use of chlorinating agents in pools was discussed as a method to help keep pools safe by killing organisms that could cause illness. Sodium hypochlorite, NaClO(s), is often added to pools to form hypochlorous acid, HClO(aq) in the water. If the water contains too much acid, it might sting your eyes and it will react with substances in the concrete walls of the pool and in the mortar between the tiles. Therefore, a proper balance must be achieved. To maintain the proper acidity in the pool, the pool manager must regularly check the pH of the water, as shown in Figure 6.7 . How acidic or basic a solution is can be described in terms of the pH. As you learned in the previous section, acids produce hydrogen ions when they dissolve in water. Hypochlorous acid is no diff erent, and ions form according to the chemical equation HClO(aq) → H + (aq) + ClO (aq) Figure 6.7 This is a simple test kit to determine the pH of the water in the pool. A pH of 7.2 to 7.6 is ideal to keep the pool clean. Notice that this test kit also checks for proper levels of chlorine, which you have learned is also a common chemical used in pool maintenance. Chapter 6 Acids and Bases • MHR 229
The pH Scale Most common acids and bases form colourless solutions. Determining the pH of a solution can help you tell whether an unknown solution contains an acid, a base, or neither. The pH scale is a scale that typically ranges from 0 to 14, which is used to classify solutions as acidic, basic, or neutral. Figure 6.8 shows the pH values of some common substances. Acidic Solutions: pH < 7 Notice that acids have pH values below pH 7. This means there are many more hydrogen ions in the solution than hydroxide ions. The lower the pH, the more acidic the solution is. So, a lemon at pH 2 is more acidic than milk at pH 6. The pH of gastric fl uids in the stomach is between 1 and 2. Basic Solutions: pH > 7 On the other end of the scale, bases have pH values above pH 7. This means there are many more hydroxide ions in the solution than hydrogen ions. The higher the pH, the more basic the solution is. So, oven cleaner at pH 13 is more basic than eggs at pH 8. Neutral Solutions: pH = 7 A solution that is neither acidic nor basic is neutral and falls in the middle of the pH scale at pH 7. This means there is the same number of hydrogen ions and hydroxide ions in the solution. Pure water has a pH of 7, as do solutions of some compounds, such as sodium chloride. Differences in pH Values Th e pH scale was suggested in 1909 by the Danish chemist Søren Sørenson as a more accurate way to describe acid concentrations in solutions. The concentration of hydrogen ions associated with a value on the pH scale diff ers from the value above it or below it by a power of 10. For example, a solution that is pH 4 has a concentration of hydrogen ions that is 10 times

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