Activity 1: Analysis of Seawater Major Chemical Concept Part A The boiling points of distilled water and salt solutions depend on the concentration of solutes and the atmospheric pressure. The greater the ionic concentration, the greater the boiling point (at constant pressure), a result of the vapor pressure lowering caused by the presence of ions in the solution. The change in boiling point can be represented by the equation
where, DT b is the difference between the boiling point of the solution and that of the pure solvent, K b is a constant that is characteristic of the solvent and is called the molal boiling point elevation constant, and m solute is the molality of the solute in the solution. Part B The freezing points of distilled water and salt solutions depend on the concentration of solute in the solution. The greater the ionic concentration, the lower the freezing point (at constant pressure), a result of vapor pressure lowering caused by the presence of ions in the solution. The change in freezing point can be represented by the equation
where, DT f is the difference between freezing point of the solution and that of the pure solvent, K f is a constant that is characteristic of the solvent and is called the molal freezing point depression constant, and m solute is the molality of the solute in the solution. Part C Seawater has a buffering effect. It resists changes in pH, in contrast to distilled water. Level General high school chemistry. Expected Student Background Part A The student should know the use and care of a thermometer and burner. Part B The student should know the use and care of a thermometer. Part C The student should know how to safely handle acid and base solutions. Time Part A Students should be able to complete Part A in one 55-min period if they immediately begin the evaporation of the seawater for Step 3. Step 4 requires an additional 55-min period. Part B Students should be able to complete Part B in one 55-min period if they immediately begin the evaporation of the seawater for Step 5. Part C Students should be able to complete Part C in one 55-min period. NOTE: The teacher can assign one third of the class to do Part A, one third to do Part B and one third to do Part C. Students can then share the data on an overhead projector, allowing completion of the laboratory in one day, if desired. The teacher can prepare data for Part A, Step 4 and the concentrated seawater ahead of time to shorten the laboratory time required. Safety Read the Safety Considerations in the Student Version. Materials (For 24 students working in pairs) Part A Nonconsumables 12 Burners and stands 12 Thermometers, 10°C to 110°C 12 Beakers, 100-mL 12 Test-tubes, 20- x 150-mm Barometer (or call the local weather bureau) Consumables Boiling chips Distilled water Artificially prepared seawater (chlorinity19.05 parts per thousand, ppt): Add to 1L of distilled water: 27.53g Sodium chloride (NaCl) 5.79g Magnesium chloride hexahydrate (MgCl 2 . 6H 2 O) 14.17g Magnesium sulfate heptahydrate (MgSO 4 . 7H 2 O) 1.82g Calcium chloride dihydrate (CaCl 2 . 2H 2 O) 1.23g Potassium chloride (KCl) 0.27g Sodium hydrogen carbonate (NaHCO 3 ) Part B Nonconsumables 12 Ring stands 12 Thermometers, 10°C to 110°C 12 Beakers, 100-mL and 400-mL 12 Test-tubes, 20- x 150-mm Consumables Graph paper Distilled water Seawater (see Part A for directions) Dry ice (solid CO 2 ), 6 lb Part C Nonconsumables 12 Beakers, 100-mL 12 Dropping bottles, Fisher universal indicator, pH range 4-10 12 Magnetic stirrers (optional) 24 Medicine droppers, or equivalent Consumables 0.1 M Hydrochloric acid, HCl (4.2 mL 12 M HCl to 500 mL solution) 0.1 M Sodium hydroxide, NaOH (2.0 g NaOH to 500 mL solution) Advance Preparation Parts A and B Prepare: Artificial seawater or obtain seawater Concentrated seawater (optional) Part C In addition to Part A and B, prepare: 0.1 M HCl 0.1 M NaOH Pre-Laboratory Discussion 1. Students should be instructed to start the evaporation of the salt solution to make the concentrated salt solution first (if it is not being provided by the teacher). 2. Review the concepts of pH, buffers and colligative properties. 3. Discuss supercooling and how it might appear on a cooling curve. 4. Review proper graphing technique including title, axes labels, proper sizing of units on each axis. 5. Warn the students not to use the thermometer as a stirring rod because the fragile bulb at the end of the thermometer could break. 6. For Part C, if you have buffer solutions from pH 4 through pH 10, it would be helpful if students could see the actual universal indicator colors shown in Figure 4. Teacher-Student Interaction Circulate in the laboratory. Ask questions regarding expected results. Post-Laboratory Discussion 1. Have the students place their freezing point and boiling point results on the board or overhead projector and determine the average class values. 2. Discuss why there is a range of values (difference in equipment, rate of heating, and rate of cooling). 3. Discuss the shapes of the cooling curves from Part B. 4. Discuss other natural buffering systems, like blood. The Chemistry of Seawater (SEAW) 11 Anticipated Student Results
Answers to Implications and applications Part A 1. The boiling point of seawater increases as the salt concentration increases: Conc. seawater > Seawater > Distilled water 2. The boiling points increase slightly as the pressure increases. 3. Yes. 4. Increasing the salt concentration increases the boiling point, by molal boiling point elevation, a colligative property (see Solutions module). Part B 1. Yes. Distilled water > Seawater > Conc. seawater. 2. The temperature stabilizes as soon as ice starts to form in the test-tube. 3. The freezing point decreases as the salt concentration increases, by molal freezing point depression, a colligative property (see Solutions module). Part C 1. The pH changes rapidly with the addition of HCl or NaOH to distilled water. Addition of even large quantities of HCl or NaOH to seawater changes the pH only slightly. 2. Seawater. 3. One example is the buffering of blood by a CO 2 /HCO 3 . (There are also other buffers in the blood.) Post-Laboratory Discussion A discussion of supercooling will probably be in order after the students have plotted their results for Part B. The teacher may wish to discuss the importance of the buffering effect of the HCO 3 /CO 3 2 in the oceans and the environment. Assessing Laboratory Learning 1. What is a buffer solution? [A solution that resists changes in its pH when either an acid or a base is added.] 2. Give an example of buffer solutions that occur in nature. [Blood and seawater (any solution that resists changes in pH).] 3. As the concentration of a salt in a solution is increased, what happens to the freezing point. Explain. [The freezing point decreases. The depression is due to the colligative properties of the system (the solution has a lower vapor pressure than that of pure water).] 4. As the concentration of a salt in a solution is increased what happens to the boiling point. Explain. [The boiling point increases. The elevation is due to the colligative properties of the system (a nonvolatile solute elevates the boiling point of the solution because the vapor pressure of the liquid is decreased).] 5. If a nonvolatile soluble substance were added to a liquid other than water, predict how its boiling point and freezing point be affected. Why? [The boiling point would be elevated and the melting point would be depressed, but not to the same degree as water since the K f and K b values are different for each solvent.]
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