Have students further explore solu- tions using Art in Motion: A Salt Solution. Check for Understanding USE VOCABULARY Divide the class into small groups, and ask students in each group to work together to create an acrostic poem based on the vocabulary term polar. (An acrostic poem is a poem in which the first letter of each line spells out another message.) Their poems should state facts about water that are due to its polarity, such as “P: Possible to dis- solve many substances easily.” ADJUST INSTRUCTION If students’ acrostics reveal they are confused by the polar nature of water, have them write two quick summary sentences. These sentences should explain how water’s polarity affects its ability to form hydrogen bonds and dissolve other polar molecules. Have pairs share their summaries and then work together to write a new acrostic poem. Answers FIGURE 2–9 The ions are surrounded by water molecules, separating the sodium and chloride in solution. Eventually, the ions become evenly distributed throughout the solution. 42 Chapter 2 • Lesson 2
Acidic and Basic Foods 1 Predict whether the food samples provided are acidic or basic. 2 Tear off a 2-inch piece of pH paper for each sample you will test. Place these pieces on a paper towel. 3 Construct a data table in which you will record the name and pH of each food sample. 4 Use a scalpel to cut a piece off each solid. CAUTION: Be careful not to cut yourself. Do not eat the food. Touch the cut surface of each sample to a square of pH paper. Use a dropper pipette to place a drop of any liquid sample on a square of pH paper. Record the pH of each sample in your data table. Analyze and Conclude 1. Analyze Data Were most of the samples acidic or basic? 2. Evaluate Was your prediction correct? Acids, Bases, and pHWhy is it important for cells to buffer solutions against rapid changes in pH?Water molecules sometimes split apart to form ions. This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction.H2O H+ + OH–water hydrogen ion + hydroxide ionHow often does this happen? In pure water, about 1 water molecule in 550 million splits to form ions in this way. Because the number of positive hydrogen ions produced is equal to the number of negative hydroxide ions produced, pure water is neutral. The pH Scale Chemists devised a measurement systemcalled the pH scaleto indicate the concentration of H+ionsin solution. As Figure 2–10shows, the pH scale ranges from 0 to 14. At a pH of 7, the concentration of H+ions and OH–ions is equal. Pure water has a pH of 7. Solutions with a pH below 7 are called acidic because they have more H+ions than OH–ions. The lower the pH, the greater the acidity. Solutions with a pH above 7 are called basic because they have more OH–ions than H+ions. The higher the pH, the more basic the solu-tion. Each step on the pH scale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10 times as many H+ions as a liter of a solution with a pH of 5.In Your Notebook Order these items in order of increasing acidity: soap, lemon juice, milk, acid rain.Oven cleanerIncreasingly BasicNeutralIncreasingly AcidicBleachAmmonia solutionSoapSeawaterToothpasteHuman bloodPure waterMilkNormal rainfallAcid rainTomato juiceLemon juiceStomach acid14131211109876543210FIGURE 2–10The pH Scale The concentration of H+ions determines whether solutions are acidic or basic. The most acidic material on this pH scale is stomach acid. The most basic material on this scale is oven cleaner.The Chemistry of Life 43LESSON 2.2The Chemistry of Life 43Use VisualsUse Figure 2–10to familiarize students with acids, bases, and pH. Point out that pH is a measure of hydrogen ion concentration. Then, explain that acids
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