1 lab period; work in pairs. Complete the Preparation page before laboratory.
Goals
Background
A sample of matter may be classified as either a pure substance or a mixture of pure substances. At the nanoscale, a mixture consists of at least 2 types of fundamental units (that is, atoms or molecules). Air is an example of a mixture. Any sample of air contains atoms of both nitrogen and oxygen, in about a 4:1 ratio. At the nanoscale a pure substance consists of only 1 type of fundamental unit. A pure substance may be classified as either an element or a compound. An element consists of only 1 type of atom; i.e., all of the atoms in a sample of an element are the same. An example of an element is copper, a reddish metal, which consists entirely of copper atoms arranged in an orderly fashion. A compound consists of at least 2 types of atoms that are connected together in specific ways so that the resulting aggregates are all the same. An example of a compound is water, in which the fundamental units are aggregates containing 2 atoms of hydrogen (an element) and 1 atom of oxygen (an element) tightly bonded (connected) together. Because there are 2 types of atoms, water is a compound, not an element; but because all the fundamental units of water are the same (2 H atoms bonded to 1 O atom), water is a pure substance!
At the macroscale, a mixture may be separated into its component pure substances using physical processes only, where a physical process is one that does not alter the identities of any of the components. A simple mixture can be prepared by stirring together a teaspoon of salt with a teaspoon of pepper. In this case, it is easy to tell visually that the result is a mixture, because both dark and white specks can be seen, and the distribution of white and black may not be uniform (even). The two components may be separated by placing the mixture in some water, which will dissolve the salt and float the pepper. The resulting mixture of water, salt, and pepper may then be passed through a coffee filter. This will retain the pepper and pass the solution of salt in water. The salt may then be recovered by evaporating the water. Filtration and evaporation are physical processes because no new substances are produced when they are carried out. That is, the salt remains salt throughout the separation sequence, the pepper remains pepper, and the water remains water. In theory, any mixture may be separated into its components using only physical processes. Most all samples of matter that we encounter on a day-to-day basis are mixtures. Examples include milk, soda, tylenol tablets, bread, cloth, and so on.
A pure substance remains unchanged by any physical process. Salt is a pure substance. No matter what physical process is carried out on a sample of salt (melting, dissolving in water followed by evaporation, grinding), only salt results. It is not possible to separate salt or any other pure substance into components using physical methods. Pure substances may, however, be altered by chemical processes. In a chemical process, rearrangements of the atoms in fundamental units occur, and new substances are produced. For example, when hydrogen and oxgyen gases are mixed and ignited, an explosion takes place and liquid water is formed. Water is a pure substance, but it is a different pure substance than hydrogen and oxygen. Conversely, water, a compound, may be separated into its component elements by a chemical process. If an electric current is applied to water in a suitable apparatus, the water molecules (2 H bonded to 1 O) are disrupted and the separated atoms recombine to form hydrogen and oxygen gases. We encounter a few pure substances on a daily basis: distilled water, diamond (pure carbon), and copper (in pennies minted before 1981, in electrical wire) are examples.
By carrying out appropriate physical then chemical processes, it is possible (in theory) to separate any mixture into its component elements. For example, a mixture of sand and salt may be separated by mixing with water to dissolve the salt; filtering to isolate the sand (a compound of silicon and oxygen); and evaporating the water to recover the salt (a compound of sodium and chlorine). At this point the original mixture has been separated into its component pure substances, both compounds, by physical methods. The salt may now be decomposed into its elements, sodium and chlorine, by melting it and passing a current through it. The sand may be decomposed into its elements by reacting it at high temperature with chlorine gas to give SiCl4 and water. The water may be decomposed with electricity to obtain the oxygen originally present in the sand. SiCl4 may be reacted with magnesium metal to produce elemental silicon, Si, and magnesium chloride.
In this experiment, you will use physical methods to resolve the components of several mixtures, carry out the chemical separation of a compound into its elements, and examine elements, compounds, and mixtures at the nanoscale.
Focus Questions
As you proceed through the experiment, keep the following questions in mind. When you have finished the experiment, provide answers to them.
Equipment and Materials
Safety
Eye protection must always be worn in the laboratory. The materials used in this experiment are not harmful; however, good lab practice requires the use of eye protection.
Experimental
Record all data in your laboratory notebook.
Separation of mixtures by physical methods: gravity and suction filtration. Obtain or prepare about 50-mL of a mixture of sand in water. Prepare for gravity filtration by folding a piece of filter paper as instructed, wetting it, and fitting it into a long-stem funnel. Mount the funnel in a ring over a clean beaker. Stir the sand/water mixture with a stirring rod, and carefully pour mixture into the funnel until the water level is near (but not over) the top of the filter paper. Continue to add mixture as the level in the funnel drops. When the gravity filtration is finished, carefully remove the filter paper from the funnel, unfold it, and place it on your clean work area so that the sand can dry.
Combine the sand and water to once again obtain a mixture. Prepare for suction filtration by assembling the suction filtration apparatus (sidearm test tube, rubber collar, Buchner funnel, and filter paper disc) and connecting it to the aspirator trap. Activate suction by turning on the water and closing the trap vent tube. Pour mixture into the Buchner funnel so as to keep the liquid level within 1/4 to 1/2 inch of the top of the funnel. Transfer as much of the mixture as you can to the funnel, then allow the filtration to finish. Empty the Buchner funnel onto a clean Kimwipe or small piece of paper towel.
When finished, return sand to the instructor.
Separation of mixtures by physical methods: distillation. Obtain or prepare about 50 mL of a solution of salt in water. Transfer the solution to a 100-mL round-bottom flask, add a boiling stone, and connect the flask to a distillation apparatus consisting of a simple distillation head, a condenser, and a receiving flask. Attach the lower inlet of the condenser to a water supply, and provide a gentle flow of water through the condenser. Heat the round-bottom flask gently with a Bunsen burner, bringing the water to a boil. Boil the mixture continuously until about 5 mL of liquid remain in the round-bottom flask, then discontinue heating. Allow the round-bottom flask to cool, then add about 10 mL of acetone. Suction filter the resulting mixture. Suction the salt dry, then transfer it to a clean watch glass.
Obtain or prepare about 5 grams of a mixture of salt and sand. Design a procedure for separating the salt from the sand and have it approved by the instructor. Then carry it out. Show the instructor the separated salt and sand when you are finished.
Separation of mixtures by physical methods: chromatography.
Thin-Layer Chromatography (TLC). Thin layer chromatography is an extremely useful technique for the analysis of very small amounts of mixtures. Please read about it before coming to class. Obtain two ink samples from the instructor, and analyze them by TLC using the procedure given.
Column Chromatography.
Distinguishing Mixtures and Pure Substances at the Nanoscale: Using Models Obtain a beaker filled with "fundamental particles" simulated using Leggo pieces, snap beads, or molecular model kits. Attempt a physical separation of the material in the beaker by removing one fundamental particle at a time. Place the fundamental particles in piles according to their structures, continuing until the beaker is empty. If at the end you have only one pile, the material is a pure substance. If you have more than one pile, the material is a mixture.
Clean-up. When you have finished all of your work:
Disposal Methods
No disposal required.
Preparation
Molecularity: Physical and Chemical Separations