Experiment 2:  SUBSTANCES AND MIXTURES OF SUBSTANCES

In this experiment you will separate a mixture of three substances - sodium chloride (NaCl, table salt), benzoic acid (C₆H₅COOH, a common food preservative), and silicon dioxide (SiO₂, sand) -  into pure substances. The separation will be accomplished by utilizing the differences in the solubilities of these materials in water. Additionally, you are asked to determine quantitatively the relative amounts of each of these substances in your sample (percent composition).

Solubility is defined as the amount of the solute that will dissolve in a given amount of solvent. The extent to which a substance dissolves depends mainly upon the physical properties of the solvent and the solute and also to some extent upon the temperature. In this case the solvent is water, a strongly polar substance, and the three possible solutes each have distinct structure and thus distinct solubility behavior.

Solubility is a physical property and (like all other chemical and physical properties) ultimately depends upon the structure and bonding of the substances involved. Exactly how chemical and physical properties depend upon the structure of molecules is a major focus of this course and is treated in detail in the textbook. Briefly, silicon dioxide is a nonpolar substance with a very high molar mass and is thus insoluble in water at all temperatures. Benzoic acid is a polar covalent compound that is slightly soluble in water. The ionic substance sodium chloride dissolves readily even in cold water. The temperature dependence of the solubility of benzoic acid and sodium chloride in water is shown in Figure 4.

Figure 4. Solubility as a Function of Temperature

This exercise includes five Pre-Lab questions which are to be answered before the laboratory period and turned in at the beginning of the laboratory period.

PRE-LAB QUESTIONS

1. Distinguish unambiguously between a mixture and a substance and give an example (not from the experiment) of each.
2. Is it possible to have a heterogeneous pure substance? If so, please give an example.
3. Classify each of the following as an element, compound, or mixture: (a) Kool-Aid, (b) helium, (c) mud, (d) cement, (e) sugar.
4. In the eighteenth century there was considerable conflict over the "discovery" of new elements. If you were president of one of the scientific societies of the day, what evidence would you find convincing that a new element had in fact been discovered?
5. Salad dressing is a mixture with which you are familiar (basically, vinegar, oil, and spices). Please outline, in good detail, a procedure by which this mixture might be separated into its component substances.

PROCEDURE

1. Carefully weigh a clean, dry 250-mL beaker to the nearest milligram (0.001 g). The various beakers that you use should be marked in some way on the white labeling spot in order to identify one from another.  Transfer about 5 to 6 g (about a tablespoonful) of the mixture into the beaker and reweigh the beaker and contents. From these two masses, the exact mass of the sample of the mixture may be calculated.
2. Add about 50 mL of distilled water to the beaker and place the beaker and contents on wire gauze on the ring stand as shown in Figure 5.

Figure 5. Set-up for heating beaker on ring stand.
3. Heat the mixture to near boiling and stir the mixture with a stirring rod to make sure that all soluble material is dissolved. At the boiling temperature, all benzoic acid and sodium chloride should be in solution (there should be no white solids visible).   If necessary, add small amounts of water until the dissolution is complete.  At this point the substances soluble in hot water have been extracted from the insoluble sand.  Use of excessive amounts of water in this and later steps will greatly increase the length of time necessary to complete the lab.
4. Place about 50 mL of distilled water in a beaker and bring it to a boil. This boiling water will be used in step 6.
5. Quickly decant the supernatant liquid in the first beaker while it is hot into another pre-weighed 250-mL beaker, guiding the solution by pouring down a glass stirring rod. Use the beaker tongs to hold the beaker of hot solution. Do not let the sand get into the second beaker.
6. In order to assure that all of the soluble materials are separated from the sand, wash the sand in the first beaker with about 15 mL of boiling water and decant the washings into the second beaker.  If necessary, use a second rinse with boiling water to get all of the soluble material.  Note the precaution in step 3.
7. Place the second beaker that now contains a water solution of benzoic acid and salt into an ice bath and let it cool. Observe the benzoic acid crystallizing out of solution.
8. Heat the sand in the first beaker with a gentle flame from a gas burner until the sand is completely dry. The sand has a tendency to splatter if heated too rapidly. The possibility of loss of sample can be reduced if the beaker is covered with a watch glass (weigh it first) and the heating is carried out very slowly.
9. When the sand is completely dry, allow the beaker to cool to room temperature. Reweigh this beaker which now contains only the sand. Calculate the mass of sand in the sample. (NOTE: You may proceed to the next step while the beaker and sand are cooling.)
10. Assemble the Buchner funnel with a filter flask and connect the filter flask to a trap and vacuum source as shown in Figure 6.  Unlike the photo, the filter flask rather than the vacuum trap should be clamped to the ring stand.

Figure 6. Filter flask set-up
11. Place a circle of filter paper in the Buchner funnel (it must lie flat), wet it with distilled water using a wash bottle, and turn on the vacuum source.
12. Swirl the contents of the second beaker and quickly pour the contents into the Buchner funnel.
13. Wash the solid (benzoic acid) with about 5 mL of chilled distilled water to ensure that the benzoic acid is free of sodium chloride.
14. Continue suction for about five minutes beyond the point at which water stops dripping from the funnel to help dry the precipitate.
15. Disconnect the rubber tubing from the filter flask before turning off the vacuum. This prevents water from backing up into the filter flask.
16. Using a spatula, transfer the suction-dried benzoic acid into a clean, pre-weighed 150-mL beaker. Let it dry further in the oven at 80^o C. Cool the beaker to room temperature and weigh it. From the total mass, calculate the mass of benzoic acid in the sample.
17. Transfer the filtrate solution from the filter flask to a clean, dry pre-weighed beaker. Using a wash bottle, rinse the filter flask into the beaker with a small amount of water to assure that all of the sodium chloride solution is transferred.
18. Place the beaker containing the salt solution on a wire gauze resting on an iron ring on the ring stand, Figure 5. Heat the solution gently with a burner so that the liquid does not boil over. Later reduce the flame to avoid overheating. It may prove helpful to cover the beaker with a pre-weighed watch glass at this point.
19. When the sodium chloride is completely dried, remove the flame and cool the beaker and contents to room temperature. Weigh the beaker and its contents to the nearest 0.001 g. From this datum and the masses of the empty beaker and the watch glass, calculate the mass of sodium chloride in the sample.
20. Calculate the percentage of each substance in the mixture. Also calculate the total percentage of the sample recovered in the operation.
21. Obtain the known percentages from your instructor and write a careful, detailed discussion of systematic errors (explain why each component is either too high or too low).
22. Place the recovered benzoic acid in the labeled waste jar, and clean up all of your equipment and put it away (except for the vacuum trap and ring stand).

(revised 09/14/2006)