LABORATORY ACTIVITY: STUDENT VERSION Note on Laboratory Activities These two activities represent only a few examples of the types of experiments that can be used to demonstrate an investigative activity in chemistry. The major concepts of scientific measurement and data analysis, chemical reactions, stoichiometry, properties of solutions, acids and bases, oxidation-reduction reactions, and solubilities, to mention a few, are represented in these two activities. Activity 1: Analysis of Seawater Introduction You will use the methods of chemical oceanography to conduct the water analysis in this investigation. Through these activities, you will solidify such concepts as the physical properties of water and pH. The investigation will be in the form of questions, and you are challenged to answer these questions using your knowledge of chemistry and the scientific method. Purpose 1. To investigate and compare some of the physical and chemical properties of seawater and distilled water. 2. To make predictions about the physical and chemical properties of seawater and distilled water (based on experiences in the high school chemistry course). 3. To test predictions. 4. To interpret data and draw conclusions based upon experimental results. Safety Wear protective goggles throughout the laboratory activity. Part A. Boiling Point The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the opposing pressure of the atmosphere. The boiling point of water is commonly given as 100 °C at one atmosphere pressure. Procedure 1. Set up the heating apparatus (Figure 1), measure and record the boiling point of distilled water. (Add boiling chips to prevent bumping.) 2. Record the barometric pressure. Repeat Step 1 with a seawater sample. 3. In a 100-mL beaker, evaporate 20 mL of seawater down to 15 mL. Determine the boiling point of this concentrated seawater using the set-up illustrated in Figure 1. 4. On a day with different weather, repeat Steps 1-3. 5. Thoroughly wash your hands before leaving the laboratory.
Data Analysis and Concept Development Make a data table containing the boiling points of distilled water, seawater, and concentrated seawater at two different atmospheric pressures. Implications and Applications 1. Are there any differences between the boiling points of the three substances measured? 2. How did a change in atmospheric pressure affect the boiling points? 3. Are seawater and fresh water affected in the same way by changing atmospheric pressure? 4. Why did increasing the concentration of salt (from Step 3) influence the boiling point? Part B. Freezing Point The temperature at which a substance becomes a solid upon cooling is its freezing point, which is identical to its melting point. For water, this temperature is assumed to be 0 °C. In this activity, we will measure the freezing point of water. Procedure 1. Arrange a thermometer in the test-tube (Figure 2). 2. Cover the bulb of the thermometer with 15 mL water. 3. Pack the test-tube in some chunks of dry ice in a 600-mL beaker. 4. Record the temperature of the water every 30 sec (from the time the tube is just placed in the dry ice), until the temperature stabilizes. 5. Repeat Steps 1-4 using 15 mL seawater. 6. Take 20 mL seawater and evaporate to 15 mL. Measure its freezing point by repeating Steps 1-4. 7. Thoroughly wash your hands before leaving the laboratory. Data Analysis and Concept Development 1. Make a data table of temperature vs. time for all three liquids. 2. Plot the data on a piece of graph paper with the temperature (°C) on the y-axis and the time (seconds) on the x-axis. Use different symbols for your data points for the three liquids (e.g., x, 0 and D) or different colors to distinguish each curve.
Figure 2. Freezing point apparatus.
Implications and Applications 1. Are there any differences in the freezing points of the three substances measured? 2. Is there any particularly unusual feature of this plot? What does it mean? What caused it? 3. How did increasing the concentration of salt (from Step 6) affect the freezing point of seawater? Part C. pH of Seawater The pH of a solution is a measure of its hydronium ion concentration [H 3 O + ]. A reading of one on the pH scale represents a high H 3 O + concentration and a very acidic solution. A very basic or alkaline solution, with low H 3 O + concentration may read near 14 on the pH scale. A neutral solution will have a pH of 7 on the scale.
Procedure Testing distilled water and seawater with acid: 1. Place 50 mL distilled water into a 100-mL beaker. Add 25 drops universal indicator, stir, and record the pH by observing the color of the solution and comparing it with Figure 4. 2. Add one drop 0.1M HCl, stir, and record the pH. 3. Repeat Step 2 until 20 drops have been added, noting the pH (color) after each drop. 4. Repeat Steps 1-3 using seawater. Testing distilled water and seawater with base: 5. Repeat Steps 1-3, with distilled water and with seawater, but use 0.1M NaOH instead of 0.1M HCl 6. Thoroughly wash your hands before leaving the laboratory. Data Analysis and Concept Development 1. Collect your data from Steps 1-5 using the following tables.
Discussion Most living organisms can tolerate only slight pH fluctuations near the neutral region of the pH scale. Under open ocean conditions, an effective pH buffering system limits seawater pH values to a narrow range between 7.5 and 8.4. This buffering system results from the interaction of dissolved carbon dioxide and water. Much of the carbon dioxide dissolved in seawater combines with water to produce a weak acid, carbonic acid. Normally, carbonic acid dissociates to produce hydrogen ions and bicarbonate or carbonate ions. These reactions can be summarized as shown below. For simplicity, we have represented H 3 O + as H + .
If excess hydrogen ions are present, the reactions above proceed to the left. [H + ] decreases, preventing the solution from becoming too acidic. If too few hydrogen ions are present, the reactions above proceed to the right, making more H + available and H2CO3 is converted to HCO 3 - and CO 3 +2 . Normally, the carbonate-bicarbonate buffering system limits large-scale fluctuations in seawater pH. Even so, in restricted water bodies such as tide pools and other areas of limited circulation, the pH of the water can vary enough to strongly affect the organisms living there. Implications and Applications 1. Compare the buffering action of the distilled water vs. that of seawater. 2. Which liquid was the more effective buffer? 3. Can you think of other buffering systems in nature?
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