Properties of Solutions
Properties of Solutions- 2 day lab
Materials: Solids: sodium chloride (NaCl) fine and coarse crystals, sucrose (table sugar, C12H22O11). Solutions: saturated sodium chloride (NaCl) and iodine tincture. Liquids: Cooking oil, mineral oil, isopropyl alcohol (rubbing alcohol) (C3H7OH).
Introduction
A solution is a homogeneous mixture in which one (or more) substance is dissolved in one or more other substances. For simplicity, we will investigate solutions consisting of one substance dissolved in one other substance. The solute is the substance that is dissolved. The solvent is the dissolving substance (also called the dissolving medium). The solvent is the substance present in the greater amount. The name of the solution is based on the name of the solute. In a “sodium chloride solution,” sodium chloride is the solute and water is the solvent.
A solution has a variable composition (ratio of solute to solvent) as more or less of the solute may be dissolved in a given amount of a solvent. A solution is a homogeneous mixture because the solute remains uniformly dispersed throughout the solution after mixing. In other words, there is only one phase.
Formation of a solution depends on the nature of the solute and the solvent. In general water, composed of polar molecules, is a good solvent for many ionic compounds which are composed of ions. Nonpolar solvents like benzene and hexane, composed of nonpolar molecules, are good solvents for other nonpolar compounds. The phrase in chemistry that captures the principle expressed in the preceding sentences is “Like dissolves like.” In other words, substances will dissolve other substances of similar polarity. In this experiment, we apply this principle to solids dissolved or not dissolved in liquids and liquids which are miscible or immiscible in other liquids.
The rate of dissolving a solute depends on:
1. the particle size of the solute.
2. whether the system is agitated (mixed) or not and how vigorously it is mixed.
3. the temperature of the system.
4. the concentration of the solute in the system.
The following terms are important:
Solubility is the amount of solute that will dissolve in a given amount of solvent at a given temperature. The terms used to describe solubility of a solid solute are: insoluble, slightly soluble, and soluble. When a liquid is added to another liquid, the term miscible is used to indicate that the liquid present in a lesser amount formed a homogeneous mixture with the liquid present in the greater. When the liquids form layers, the term immiscible is used.
Concentration of a solution may be expressed in words or mathematically as a ratio. It expresses the ratio of solute to solvent in a solution. The following word expressions are common and important to know:
Dilute solution: A solution that contains a small amount of solute per unit volume of solution.
Concentrated solution: A solution that contains a large amount of solute per unit volume of solution.
Saturated solution: A solution that contains the maximum amount of dissolved solute possible at a given temperature. In the laboratory, saturated solutions appear as homogeneous liquids on top of undissolved solid particles. The liquid, called a “supernatant liquid” because it is over (above) the undissolved solid, is in dynamic equilibrium with the undissolved solid.
Unsaturated solution: A solution containing less solute per unit volume than the corresponding saturated solution
Supersaturated solution: A solution that contains more dissolved solute than is normally present in the corresponding saturated solution at a given temperature. The supersaturated solution is not stable and will crystallize if the solution is disturbed.
The following mathematical expressions of concentration are important to know:
Mass percent = x 100%
Molarity =
Procedure
A. Concentration of a saturated solution (Plan ahead- this needs to set overnight)
1. Preparation of the saturated NaCl solution: Weigh 36.6 g of fine NaCl crystals and place the crystals in a coffee mug. Add 100.0 mL of water to the mug. Place the mug in the microwave and heat the solution until it boils. Stir until all (or the great majority) of the crystals dissolve. Let the solution cool.
2. Weigh a small bowl (needs to weigh no more than 50-60 g). Record the mass.
3. Transfer 6 mL of the saturated NaCl solution to the bowl. Make sure to only transfer liquid- leave any solid residue behind. Weigh the bowl with the solution. Record the mass.
4. Allow the bowl to set out overnight or until all the liquid has evaporated. Reweigh. Add water to the solid to dissolve it and dispose of it in the sink.
B. Relative solubility of a solute in two solvents
1. Add 2mL of cooking oil and 2mL of water to a test tube, stopper it, and shake gently for 5 seconds. Allow the layers to separate and note which liquid has the greater density.
2. Add 5mL of iodine tincture to the same test tube, note the color of each layer. Stopper and shake gently for 20 seconds. Allow the liquids to separate and note the color of each layer.
3. Dispose of the mixture and clean the test tube.
C. Miscibility of liquids
1. Take 3 dry test tubes and add liquids as follows:
a. 1mL mineral oil and 1 mL isopropyl alcohol to test tube #1.
b. 1mL mineral oil and 1 mL water to test tube #2.
c. 1 mL water and 1 mL isopropyl alcohol to test tube #3.
2. Stopper each test tube and shake for about 5 seconds. Note which pair is miscible.
Dispose of waste in the designated container.
D. Effect of particle size on rate of dissolving
1. Fill a dry test tube to about 0.5cm deep with fine crystals of sodium chloride. Fill another test tube to about the same depth (0.5cm) with coarse sodium chloride crystals.
2. Add 10mL of tap water to each test tube and shake each tube. Note the number of seconds required to dissolve the salt in each test tube.
E. Effect of temperature on rate of dissolving
1. Weigh two 0.5g samples of fine sodium chloride crystals.
2. Add 50mL of cold tap water to a 100mL beaker. Add 50mL of boiling water to a 250mL beaker.
3. Add the 0.5g of sodium chloride to each beaker and slowly tilt the solutions in each beaker. Observe how fast the sodium chloride dissolves in each solution. (Do not stir the solutions)
F. Solubility versus temperature; saturated and unsaturated solutions
1. Label 4 weighing boats or papers as follows and weigh the stated amounts onto each one:
a- 1.0g NaCl b- 1.4g NaCl c- 1.0g Sucrose d- 1.4g Sucrose
2. Place each 1.0g sample into a separate labeled test tube. Add 5mL of tap water to each, stopper, and shake until the solid in each test tube dissolves.
3. Add 1.4g NaCl to the NaCl solution of part F.2. Add 1.4g Sucrose to the Sucrose solution of part F.2. Stopper and shake both test tubes. Note whether the crystals have dissolved in each.
4. Place both unstoppered test tubes into a 500 mL Beaker with boiling water and stir frequently and vigorously, being careful not to break the bottom of the test tubes. Note results after 5 minutes. (You will need to continually add boiling water to the beaker so that the test tubes are consistently heated.)
5. Cool the test tubes in running water for about a minute and let them stand for a few minutes. Record your observations.
Name__________________________
Section _______ Date _____________
Report for Experiment 11 Instructor_______________________
A. Concentration of a saturated solution
1. a- Mass of evaporating dish __________________
b- Mass of evaporating dish and saturated NaCl solution __________________
c- Mass of dish and NaCl after evaporation __________________
2. Calculate: (Show work. For more space, use the reverse side.)
a- Mass of saturated NaCl solution __________________
b- Mass of NaCl dissolved in the solution __________________
c- Mass of water in the solution __________________
d- Mass % of NaCl in the solution __________________
e- Grams of NaCl per 100g of water __________________
B. Relative solubility of a solute in two different solvents
1. Which liquid is more dense, water or cooking oil? __________________
2. What evidence supports your answer? ____________________________________
3. What is the color of iodine in water? __________________
4. What is the color of iodine in cooking oil? __________________
5. In which solvent was iodine more soluble? __________________
6. What is the experimental evidence? _____________________________________
__________________________________________________________________
C. Miscibility of liquids
1. Which liquid pair(s) tested is miscible? __________________
2. How do you classify the water/mineral oil mixture? __________________
D. Rate of dissolving versus particle size
1. Time (seconds) required for fine salt to dissolve __________________
2. Time (seconds) required for coarse salt to dissolve __________________
3. What general conclusion can you draw?
_______________________________________________________________________
E. Rate of dissolving versus temperature
1. Did salt dissolve faster under hot or cold conditions? __________________
2. Length of time (seconds) for salt to dissolve in hot water __________________
F. Solubility Versus Temperature
1. Which of the 1.0g solutions is saturated? __________________
2. Evidence? _________________________________________________________
3. Which of the 2.4g solutions is saturated? __________________
4. Evidence?_________________________________________________________
5. Which solid is least soluble at the elevated temperature? __________________
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