MIXTURES, COMPOUNDS, & SOLUTIONS As with elements, few compounds are found pure in nature and usually found as mixtures with other compounds. A mixture is a combination of two or more substances that are not chemically combined and do not exist in fixed proportions with each other. Air is an example of a mixture of oxygen gas and nitrogen gas uniformly mixed in the atmosphere. Soft drinks, spring water, as well as solids such as the alloys brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin) are uniform mixtures called homogeneous mixtures. A homogeneous mixture has the same uniform appearance and composition throughout. Many homogeneous mixtures are commonly referred to as solutions. Heterogeneous mixtures consists of visibly different substances or phases. The ingredients of a pizza, the composition of sand, and the grainy appearance of wood indicate that these are not uniform in composition. Matter No Is the composition uniform throughout? Yes Heterogeneous Mixture Homogeneous No Can it be separated by physical processes? Yes Pure Substance Homogeneous Mixture (Solution) Can it be broken down into other substances by chemical processes? No Element Compound Yes Mixtures can be separated by physical means to isolate or purify their components if they have different chemical or physical properties. A physical separation does not change the chemical properties of the components which compose the mixture. Distillation separates liquids based on their boiling points while filtering separates solids based on their particle size or their solubility in a liquid. Components of a mixture may also be separated on whether they sink or float in water which is a funtion of their densities. PART I: SEPARATION OF MIXTURES USING CHROMATOGRAPHY The purpose of this part of the laboratory is to use paper chromatography to see if the color of water-soluble marking pens results from a single dye or mixture of dyes. This technique uses a solvent that moves up the paper by capillary action. When the solvent reaches the spot of the mixture, the solute, the components dissolve in the solvent and are carried up the paper by the solvent. Each component will have its own characteristic balance of attractions to the solvent and to the paper, so each component will not move at the same speed. Eventually this difference in speed will separate the components. The dye molecules of the pens may be different sizes (large or small), different masses (heavy or light), or different solubilities (more or less soluble). For example, large and small molecules can be separated because small molecules travel almost as quickly as the solvent molecules and therefore get carried to the top of the chromatogram together. The largest solute molecules travel very slowly and stay near the bottom.
Materials: Two strips of chromatography paper, one unknown strip, three test tubes, 3 clips, test tube rack, water soluble pens, scissors, pencil, ruler. Color Procedure: 1. Using scissors cut the ends of the three chromatography strips to a point. On one chromatography strip place a small spot of ink with one of the colored pens 1 cm from the bottom of the strip and record the color you spotted on the strip near the top using a pencil. 2. On a second strip, spot with a different colored pen and record the color you spotted on the strip. 3. The third strip is an unknown, spotted with two different colored pens. You will determine the colors of the pens used to spot this unknown by comparing known strips spotted by either you or a classmate. 4. Add approximately 0.5 cm of water to each test tube. Suspend the chromatography strip in the water using a clothespin. Make sure the spot is above the water level. Water level 5. The chromatography strips will require about 45 minutes to develop. When the water level has climbed to within 5-6 cm from the top, remove the strips from the test tube and allow them to air dry. Identify the combination of pens used to spot your unknown chromatogram. Compare the separation of dyes in your unknown chromatogram with several known chromatograms in the room produced by your classmates. Unknown # Pen Colors: What observations lead you to identify your unknown? Location of dyes along the paper The abundance of dye In chromatography, the size of the molecule, the shape of the molecule, and the solubility of the molecule in water affects the distance the molecule travels up the paper. For the following three conditions, circle the molecule in each pair that would migrate the farthest up the paper. a. A large molecule vs. a small molecule. b. A heavy molecule vs. a light molecule. c. A water insoluble molecule vs. a water soluble molecule. In analyzing your chromatogram and comparing chromatograms with other students, what color dye is composed of molecules which may be either large, massive, or insoluble in water? On what evidence are you basing your conclusions? The further the dye traveled up the paper strip, the smaller, lighter, or more soluble the dye is in water.
PART II: SEPARATION OF MIXTURES BY DISTILLATION QUESTION: Impure water can be purified by: (a) removing the impure water molecules; (b) removing everything that is not water; (c) breaking down the water to its simplest components; (d) adding some disinfectant such as chlorine. ANSWER: (b) Water, H 2O, is a chemical compound made of the elements hydrogen and oxygen in a 2 to 1 ratio. Every H 2O molecule is exactly the same and there's no such thing as an impure H 2O molecule. Just about anything including chemical compounds can be found in water. When something other than water is found in water we say that the water is not pure, in other words, a mixture. It is important to see that the impurities are in the water and not part of the water, which means that it is possible to remove them by a variety of physical means, like filtration or distillation. All naturally occuring water contains dissolved substances, which it takes from the rocks and soils over which it flows or from the air as in the case of rain water. To purify a sample of water it is necessary to boil it and condense the steam. This process is called distillation. Solids and liquids whose boiling temperatures are much higher than that of water are left behind. Substances having lower boiling temperatures than water, like alcohol, are distilled before the water. Procedures: Add 25 ml of colored water solution to the distillation tube. IMPORTANT: Add two (2) boiling stones which keeps the boiling proceeding at an even rate. Set up the apparatus shown in the diagram. DO NOT REMOVE THE DISTILLATION TUBE FROM THE SAND. Place the thermometer and stopper securely in the top of the distillation tube. Attach the condensing column to the distillation tube and rest the end on top of a graduated cylinder. Wrap two paper towels soaked in water around the condensing column. This keeps the column cool so that the water vapor condenses to a liquid. Turn the hotplate to #10. When the solution begins to boil, turn off the hot plate. The colored water will continue to boil. Collect the distilled water in the graduated cylinder. Note the color of the water collected. Allow everything to cool. Thermometer Stopper Can with sand Distillation tube with colored water Condensing Tube Graduated cylinder Observations of the colored water solution and distillate: The water was colored red in the distillation tube. The collected water in the graduated cylinder (the distillate) is colorless. The dye has a boiling point greater than water so it did not boil along with the water.
PART III: CHANGING PHASES Normally, if you give a substance, such as water, a constant amount of heat, its temperature will rise at a constant rate. This rate is dependent upon the specific heat of the substance. However, this does not occur during a phase change. When a substance is being heated through a phase change (solid to liquid or liquid to gas), the temperature does not change, instead, it stays constant until the phase change is complete. Thus, all the heat that is given to the substance during a phase change is known as latent heat or "hidden" heat. It is hidden since there is no observable change in temperature with the addition of heat. Instead of changing the temperature the heat during a phase change is used to change the phase of the substance. If a substance is being cooled instead of being heated a similar situation will occur where the temperature of the substance doesn't change during the phase change. PROCEDURE: 1. Fill the 600 ml beaker about 2/3 full of water and place it on the hot plate. 2. Obtain a test tube of frozen water with a temperature probe from the instructor. 3. Insert the test tube in the holder and immerse the test tube in the water. 4. Connect the temperature probe to the recording device. Press the button on the data collector to begin recording and graphing the temperature. 5. Turn on the hot plate to #7. 6. Continue heating and recording the temperature in the test tube until the water in the beaker has boiled for approximately 3 minutes. 7. Press the button on the data collector to stop collecting data. Turn off the hot plate and remove the test tube and place it in the test tube rack. 8. Press the Autoscale button to display the entire range of data. Draw the plot of time vs temperature on the graph below. 100 o 0 o Time
ANALYSIS Label the portions on the graph where the temperatures are increasing and where the temperatures are constant. At the molecular level, what is happening to the water molecules when the temperature is increasing? Increasing kinetic energy and velocity. At the molecular level, what is happening to the water molecules when the temperature is constant? Intermolecular bonds between water molecules are being affected. During melting, the tight and rigid bonds between water molecules are weakening. This allows water molecules to flow as a liquid. During boiling the intermolecular bonds are being broken resulting in free water molecules in the gaseous phase. Water boils at 100 C (under standard conditions). However, as the water continues to heat, the water temperature does not increase. What is the heat being used for? Breaking intermolecular bonds between water molecules. PART IV: EFFECTS OF TEMPERATURE ON SOLUBILITY Solutions are homogeneous mixtures. The most common solutions are liquids; for example, alcohol dissolved in water, and salt dissolved in water. The substance in excess in a solution is called the SOLVENT, and the substance dissolved is the SOLUTE. A solution in which the solute is present in a small amount relative to the solvent is a DILUTE solution; if a relatively large amount of solute is dissolved, it is a CONCENTRATED solution. When a solute is added to a solvent, it dissolves, and if thoroughly stirred, the distribution of molecules is the same throughout the solution (A HOMOGENEOUS MIXTURE). If more solute can be dissolved in the solution at the same temperature, it is said to be an UNSATURATED SOLUTION. As more solute is added, the solution becomes more and more concentrated. If the maximum amount of solute is dissolved in the solvent, then the solution is SATURATED. Solubility depends on the temperature of the solution. If the temperature of the solvent is raised, the solubilities of practically all solutes increases. Hot water, for example, dissolves more solute than cold water. However, the opposite is true for gases. If a gas such as CO 2 (carbon dioxide) is dissolved in water, the solubility of the gas is less in warm water than in cold water. If an unopened soft drink is allowed to warm, the solubility of CO 2 decreases. When the bottle is opened, CO 2 may escape so fast that the beverage shoots out of the bottle. When solutions of solids in liquids are prepared at high temperatures and then cooled, solubility drops and may reach the saturation point, where excess solute begins crystallizing from the solution. However, if no crystals of the solid are present in the saturated solution, crystallization may not take place if it is carefully cooled. The solution will then contain a larger amount of solute than the maximum and the solution is said to be SUPERSATURATED. The addition of a "seed" crystal will cause the excess solute to crystallize.
PROCEDURE: A saturated solution of the compound sodium thiosulfate has been prepared for you. Place the test tube in the hot water bath (used in Part III above) until the solid has completely dissolved. Remove the test tube and place it in an ice water bath. Let the solution cool for about 10 minutes, then remove the test tube and place it in the test tube rack. Remove a "seed crystal" from the vial with the forceps and add this to the supersaturated solution. WATCH WHAT HAPPENS! Feel and compare the temperatures of the liquid and solid portions in the test tube. ANALYSIS: At what point in this experiment is the solution saturated? What observations lead you to this conclusion? Prior to placing the test tube in the hot water there was the presence of solid (undissolved) solute. This is when the solution is saturated. You can tell a saturated solution because undissolved solute exists in the solution. Similar to undissolved sugar at the bottom of an iced coffee. At what point is the solution supersaturated? What observations lead you to this conclusion? After the solution was heated and placed in the ice water bath. Decreasing the temperature should have decresased the solubility of the solute. It wanted to crystallize. We can tell a supersaturated solution by placing a seed crystal in the solution. If the solution crystallizes (solidifies) immediately, then the solution is supersaturated. A solution with an unknown concentration of a solute is tested by adding a seed crystal. For each of the following possibilities, determine if the solution is either saturated, unsaturated, or supersaturated, and provide an explanation: a) The seed crystal dissolves. If the seed crystal dissolves, then the solution was unsaturated. b) The seed crystal sinks to the bottom of the solution and does not dissolve. If the seed crystal does not dissolve, then the solution is saturated. c) The seed crystal causes the entire solution to crystallize and form a solid. If the seed crystal causes the solution to crystallize, the solution was supersaturated (prior to adding the seed crystal). After adding the seed crystal the solution is saturated.