CHEMISTRY 161 – EXPERIMENT 5
Revised 12 January 1995
WATER AND AQUEOUS SOLUTIONS: How Water Creeps into Your Life
INTRODUCTION
PART I: WATER: Pure & Otherwise
Except for oxygen, water is the most important substance known to man. It accounts for two thirds of our body weight and plays an essential role in digestion, circulation, elimination and regulation of body temperature. All cellular activities take place in an aqueous or watery environment.
Water is a polar molecule and following the general rule of "like dissolving like", will dissolve only other polar materials. This helps nonpolar hydrocarbon materials such as gasoline, oil and fats do not. The polarity of the water molecule also accounts for its surface tension. Surface tension has the effect of a thin, elastic membrane at the water's surface. When nonpolar materials such as dirt, oil, fats or air come in contact with the surface of water, it is difficult for them to penetrate this seemingly elastic membrane. Thus, detergents are needed to wash clothes and bile is needed for fat digestion. Both help emulsify or mix nonpolar materials with water.
Water is often found as an integral part of many salts; these salts are called hydrates. Most hydrates are stable when exposed to air at room temperature; however, some lose water when exposed to air. They are said to be efflorescent, whereas other salts will absorb moisture from the air and are said to be deliquescent or hygroscopic.
Tap water is not 100% pure H2O, but contains many dissolved materials such as gases and ions. Hard water contains calcium, magnesium and/or iron ions. These ions interfere with the use of soap and are responsible for the "ring around the tub" syndrome. When hard water is boiled a salt deposit is left behind causing discoloration of the kettle. To soften hard water, these ions must be removed. Water softening units accomplish this by replacing the offending ions with sodium ions. However, people on low-sodium diets must be careful of water softened by this method.
In this experiment you examine some of these properties of water and test tap water and deionized (or distilled) water for the presence of some of the dissolved materials.
PARTII: AQUEOUS SOLUTIONS
Although there are many different types of solutions, only aqueous (water) solutions will be examined in this experiment. This is because water is the most important biological solvent. Blood and urine are two examples of water solutions that occur in the body. Materials travelling to and from the cell must use a watery medium. Many medications are given as water solutions.
There are two parts to a solution: the solute, the material that has dissolved, and the solvent, the dissolving medium. There are a variety of ways to express solution concentration (the ratio of solute to solvent). Dilute means a small amount of solute dissolved in a solvent, whereas concentrated means a large amount of solute dissolved in a solvent. Saturated solutions contain all the solute they can hold at a given temperature. Unsaturated solutions contain a smaller amount of the solute than they can normally hold under the given conditions. Sometimes it is possible to dissolve an excess amount of solute in a solvent. This situation produces a supersaturated solution. Such solutions are not stable and if a surface for crystal formation is provided, the excess solute will separate from the solution. All of the preceding terms, however, are relative and are not precise. In this experiment you will examine two exact methods of expressing solution concentration: percent by weight-volume and molarity. Look up their definitions in your text and be sure that you are familiar with them before attempting the calculations at the end of the experiment.
Chemical and pharmaceutical firms frequently supply solutions that must be diluted before use. The following equation can be used to solve dilution problems as long as the concentration units remain the same.
(VOLUMEinitial) (CONCENTRATIONinitial) = (VOLUMEfinal) (CONCENTRATIONfinal)
Suppose that you have 200 mL of a 5% glucose solution and you want to dilute it to 2%.
(200 mL) (5%) = (Vf) (2%)
(200ml)(5%)
2%
500 mL = Vf
You would take the 200 mL of 5% glucose and dilute it to a total volume of 500 mL.
In addition to studying solution concentrations, you will also look at several properties of solutions and at some factors involved in the solution process.
PROCEDURE
PARTI:
A. Efflorescence and Deliquesence
1. At the beginning of the lab period, place a few crystals of sodium sulfate decahydrate, Na2SO4•10H2O, on one watch glass and a few crystals of Calcium Chloride, CaCl2 on another. Visually examine the crystals and record your observations on the Data Sheet. Set the watch glasses aside and allow the chemicals to be exposed to the air until the end of the laboratory period.
2. When you finish the rest of the experiments again visually examine the crystals and record your observations. What conclusions can you draw from your observations?
B. Surface Tension
1. Obtain four clean test tubes and place about 5 mL of the following liquids into separate tubes:
Tube 1: Deionized water
Tube 2: 0.1% sodium chloride
Tube 3: 0.1% Ivory liquid soap
Tube 4: 0.1% bile salts
2. Set the tubes into a rack and carefully dust a small amount of sulfur onto the surface of each liquid. Tap the sides of each tube and record your observations. Which substances allow sulfur to break through the surface of the liquid?
C. Water of Hydration
1. Obtain two clean, dry test tubes. Add about 2 grams of sodium chloride, NaCl, to one tube and about 2 grams of copper (II) sulfate pentahydrate, CuSO4•5 H2O, to the other. Using a utility clamp to hold the test tube at an angle, heat the area of the tube that contains the chemical with a Bunsen burner for about 3-5 minutes. Watch for condensation in the cooler areas of the tube and record your observations and conclusions on the Data Sheet.
D. Dissolved Chloride
The test for chloride ion, Cl-, involves a precipitation reaction with silver ion, Ag+. This is a qualitative test rather than a quantitative test. We can determine if Cl- is present but we cannot determine the amount of Cl- that is in the sample.
1. Place about 5 mL of Tap water in a clean test tube.
2. Add about 3 drops of silver nitrate, AgNO3, solution to the water sample.
CAUTION: Silver nitrate will cause your skin to turn black!!
3. A white cloudness indicates the presence of Cl-. Record your observations.
4. Repeat this procedure using a clean test tube and deionized water in place of tap water.
E. Dissolved Inorganic Phosphate
The test for phosphate ion, PO43-, involves the use of a coloring reagent and a color comparator. Use the following instructions to determine the phosphate content of tap water and deionized water.
1. Fill both of the color viewing tubes to the 5 mL etched line with the tap water provided.
2. Open one PhosVer 3 Powder Packet and add the contents of the packet to one of the tubes. Stopper the tube and mix for two minutes to allow the color to develop. If phosphate is present, a blue-violet color will develop.
3. Insert the prepared test solution into the right top opening of the color comparator.
4. Insert the blank untreated water sample into the left top opening of the comparator.
5. Hold the comparator up to the light source and view through the two openings in the front. Rotate the disc to obtain the best color match and record the scale reading on the Report Sheet.
6. Finally, divide the scale reading by 10 to obtain the mg PO43-/L.
7. Rinse both viewing tubes with deionized water and repeat this procedure using deionized water in place of tap water.
F. Hardness of Water
Although hardness in water can be caused by Ca2+, Mg2+ and/or Fe2+,Fe3+, the most common cause is Ca2+. This test will quantitatively determine the amount of calcium in a sample.
1. Fill the measuring tube level full of the tap water provided and pour it into the mixing bottle.
2. Add one level measuring spoonful of Hardness Reagent to the mixing bottle and swirl to dissolve.
3. Using the dropper provided, add the Hardness Titrant Solution to the mixing bottle a drop at a time. While swirling the bottle and counting the drops, continue the dropwise addition of the Titrant Solution until the color changes from pink to blue.
4. Finally, multiply the number of drops used by 13 to obtain the mg Ca2+ / gal.
5. Rinse the mixing bottle with deionized water and repeat this procedure using deionized water in place of tap water.
PART II:
A. Diffusion, the Dissolving Process
1. Put about 100 mL of deionized water in a 250 mL beaker. Carefully drop one or two crystals of potassium permanganate, KMnO4, into the water. Without moving the beaker, occasionally observe the contents of the beaker during the laboratory period.
B. Supersaturated Solutions
1. Place about 5 grams of sodium thiosulfate, Na2S2O3 in a test tube and add 15 drops of deionized water. Place the test tube in a boiling water bath until the crystals are completely dissolved.
2. Place the test tube aside and without moving it, allow it to cool to room temperature (Wait until you finish the rest of the experiments).
3. Add one crystal of sodium thiosulfate to the tube and record your observations.
C. Effect of Stirring and Surface Area on Rate of Dissolution
1. Obtain two clean test tubes. Place about 0.5 grams of finely ground copper (II) sulfate, CuSO4, into each of two test tubes. Add about 5 mL of deionized water to each tube and place them in a beaker that contains boiling water. Stir the contents of the first tube, but not the second tube. Continue heating and stirring (only tube 1) until you can determine the relative rates of dissolution.
2. For the second phase of Part C, weigh one large crystal of CuSO4 and place it into a clean, dry test tube. Now measure out a similar mass of finely ground CuSO4 and place it in a second test tube. Add about 5 mL of deionized water to each tube and place them in a beaker that contains boiling water. Stir the contents of both tubes. Continue heating and stirring until you can determine the relative rates of dissolution.
D. Saturated Solutions
1. Put about 3 mL of saturated sodium chloride, NaCl, solution into two different test tubes. Add a few crystals of sodium chloride to the first and a few crystals of sugar to the second. Mix the tubes and record your observations.
E. Solubility
1. Put about 1 gram of sodium carbonate, Na2CO3, into a 100 mL beaker and add 10 mL of deionized water. Stir. If all the sodium carbonate does not dissolve, continue adding 10 mL portions of deionized water until it does dissolve or the beaker is full. Record your observations. Repeat the experiment using 1 gram of calcium carbonate, CaCO3.
F. Heats of Solution
1. Put 5 mL of deionized water into a test tube. Rapidly dissolve about 1 gram of ammonium chloride, NH4Cl, in the water and qualitatively determine any change in temperature. Record your observations.
2. Repeat the experiment using 1 gram of sodium hydroxide, NaOH.
CAUTION: Do not touch the NaOH with your fingers as it will cause severe burns.
POST-LAB QUESTIONS:
PART I:
1. Diagram the structure of the water molecule, showing its polar nature (you will need to draw the molecular geometry & use lone pairs)
3. What is the source of Cl- ions in Municiapal Tap water?
4. Under what conditions should patients avoid drinking softened water? Why?
5. Is rain water hard or soft? Explain your answer!
PART II:
1. How many grams of sodium chloride do you need to prepare 250 mL of a 5.00%(w/v) sodium chloride solution?
2. How many mL of the above 5.00%(w/v) solution are needed to make 100 mL of physiologic saline (0.90%(w/v) sodium chloride)?
3. How many grams of calcium nitrate, Ca(NO3)2, are needed to make 100 mL of a 0.5 M solution?
Name______
REPORT SHEET
WATER AND AQUEOUS SOLUTIONS
PART I:
A. Efflorescence and Deliquescence
Observations:
Sodium sulfate
Calcium chloride
Conclusions:
B. Surface Tension
Observations:
Tube 1: Deionized water
Tube 2: 0.1% sodium chloride
Tube 3: 0.1% Ivory liquid soap
Tube 4: 0.1% bile salts
Conclusions:
C. Water of Hydration
Observations:
Sodium Chloride
Copper Sulfate
Conclusions:
D. Dissolved Chloride
Observations:
Tap Water
Deionized Water
Conclusions:
E. Dissolved Inorganic Phosphate
Observations: Tap Water Deionized Water
Comparator Scale Reading ______
mg PO43- / L ______
F. Hardness of Water
Observations: Tap Water Deionized Water
Drops of Titrant Solution added ______
mg Ca2+ / gal ______
PART II:
A. Diffusion, The Dissolving Process
Observations:
Explain what is happening:
B. Supersaturated Solutions
Observations:
Explain your observation:
C. Effect of Stirring and Surface Area on Rate of Dissolution
1. Does the CuSO4 in the stirred or unstirred test tube dissolve first? Explain your observation!
2. Does the CuSO4 sample with the smaller or larger surface area dissolve first? Explain your observation!
D. Saturated Solutions
Observations:
Explain your observations:
E. Solubility
Observations:
What can you say about the relative solubilities of these two carbonates?
F. Heats of Solution
Observations:
Which reaction is exothermic? Which is endothermic?
5-1