Experiment 15-Properties of Solutions

Transcription

1 A solution is a combination of a solvent (major component) and a solute (minor component. Solutions are heterogeneous mixtures. They may contain more than one solute. Solutes can contain solid, liquid or gaseous solvents and solutes. The properties of solutions are important in chemistry. Some the important properties include solubility, conductivity, and viscosity. Viscosity will be explored in the next unit. Solubility. The solubility of a solvent depends on its polarity, size, and molecular mass. Solutions that contain a maximum amount of solute are said to be saturated. When solutions are heated, the solubility of the solute usually increases. Some solutions have even more solute dissolved in them. This is achieved by heating the solution, dissolving a large amount of solute, and then letting the solution cool. After cooling the solution, it is considered to be supersaturated. Supersaturated solutions are unstable. A simple jarring of the container may induce rapid crystallization. In this experiment we will investigate a supersaturated solution of sodium acetate prepared by heating the solution and letting it cool to room temperature. Care should be taken not to bump or disturb the solution. Conductivity. Liquids that contained charged particles (ions) have the ability to conduct electrons in the form of electric current. Even highly polar solvents like water are poor conductors of electricity without the presence of fully charged ions. In the second part of this experiment you are going to investigate the presence of ions using conductivity measurements. We will measure conductivity using two wires connected to an electric light bulb. Care must be taken not to touch the wires when electric current is turned on. You can be electrocuted. Your first test will be to place distilled water in a beaker and place the electrodes in the water. No conductivity should be detected (the light bulb should not go on when the current is on). You then will add several substances to separate beakers containing distilled water. Some substances may be dissolved in water without forming ions. They will not light up the light bulb even though they have mixed or dissolved. Many substances dissolve only by forming ions in water. Some molecular compounds will dissolve, such as H 2 SO 4 and HCl, and the molecules split apart to form ions. The remainder of the molecular compounds remain either attached in an insoluble form (CS 2 or ether) or as dissolved undissociated molecules (sugar). Those that only

2 94 partially split apart to form ions are called weak electrolytes and they weakly light up the light bulb. Those that completely split apart to form ions are called strong electrolytes and brightly light up the light bulb. If one of the ions formed is the hydrogen ion or the hydroxide ion, we say that the substance is a strong or weak acid or base depending on how much of it splits into ions and conducts electricity. Experiments Materials: 20g sodium acetate, medium 300 ml beaker, hot plate or setup to use burner, matches, distilled water, 3 large test tubes, spatula Hypothesis: Can you supersaturate water? Part A: Supersaturation 1. Measure approximately 20 g of sodium acetate into a clean and dry beaker. 2. Add 10 ml of distilled boiling water and stir. Be careful not to contaminate the solution with dirty equipment. If the solution does not dissolve, try heating on a hot plate. All of the solid material must dissolve. You may add a few ml of distilled water if the solution still has trouble dissolving. 3. If the solution does not dissolve, try heating on a hot plate. All of the solid material must dissolve. You may add a few ml of distilled water if the solution still has trouble dissolving. 4. Prepare 3 clean and dry medium sized test tubes. 5. Pour equal portions of the clear solution into the 3 test tubes. Place the test tubes in a holder or a beaker on the lab bench where it will not be disturbed. Do not bump or touch the test tubes for at least 1 hour. 6. Add a few sodium acetate crystals to one of the test tubes that still contain a clear solution. Observe the induced crystallization and write and draw your observation on the Lab Report. 7. You can repeat with the other test tubes or try just pouring one test tube onto the surface of your lab table. Some of your test tubes may already have crystallized.

3 Part B Conductivity of Electrolytes 95 Materials: (setup in fume hood) 3 small beakers, and distilled water, electric light bulb conductivity meter, solid NaCl, sugar, 6M HCl, concentrated ammonia, glacial acetic acid, solid CuSO 4 or K s Cr 2 O 7 or other colored solute), scoopula Hypothesis: What solutes are electrolytes? 1. Fill a small beaker about half full with distilled water, insert the electrodes of the light bulb apparatus and turn the power on. Record your observation on your lab report. Be sure to turn the power off when not in use. Indicate if it is a strong, weak or non-electrolyte. 2. Add a small scoop of NaCl to the distilled water in step one, insert the electrodes and turn the power on. Record your observations. Indicate if it is a strong, weak or non-electrolyte. 3. Dissolve a small scoop of sugar in a small beaker about half full of distilled water. Test the conductivity, write your observations on your lab report and indicate if it is a strong, weak or non-electrolyte. 4. Dissolve about 1 ml of dilute HCl in a small beaker about half filled with distilled water and test as before. 5. Repeat with about 1 ml of concentrated ammonia. Keep the concentrated ammonia in the fume hood to avoid strong off odors. 6. Repeat with about 1 ml of glacial acetic acid. Keep the concentrated acid in the fume hood to avoid strong odors. 7. Dissolve a small scoop of another salt or substances provided by your instructor in a small beaker half filled with distilled water and test as above.

4 96 Part C Surface Tension Materials: Petri dish or watch glass (or beaker), small paper clip or straight pin, dropper bottle of soap solution Hypothesis: Can surface tension suspend a paper clip? 1. Clean a beaker or watch glass thoroughly and fill with distilled water. 2. Carefully place a straight pin or small paper clip on the surface so that it floats. 3. Put 1 drop of soap on the surface next to the pin but not touching it. 4. Record observations in your lab report.

5 97 Prelab Exercises Name: Section: Mark True(T) or False(F) on for each of the following questions. 1. All strong and weak electrolytes are ionic compounds or salts. 2. All non-electrolytes are not ionic compounds or salts. 3. All solids that dissolve in water are ionic compounds. 4. Pure water is a good conductor of electricity. 5. It is safe to use electrical appliances while you are bathing. 6. Supersaturation is a stable condition.

6 98 Lab Report Part A Solubility-Supersaturation Draw a picture of the crystallization in your three test tubes: Test Tube 1 Test Tube 2 Test Tube 3 Describe what you saw when you added the crystals of sodium acetate:

7 Part B Conductivity Conductivity Scale A B C D F bright dark 99 Type of Electrolyte Conductivity Strong Weak Non distilled water NaCl HCl NH 4 OH HC 2 H 3 O 2 sugar salt salt Part 3 Surface Tension Observations when soap is added:

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