A Student Experiment and Lecture Demonstration ~ i x t u r eVersus Compound ALLEN SCATTERGOOD Union Junior College, Cranford, New Jersey
T
HE DIRECT combination of iron and sulfur, a standard experiment used to illustrate the difference between compounds and mixtures, is open to many criticisms. It is difficult to show that iron is a substance, since its melting point cannot be determined with the usual laboratory equipment. It is impossible to show that iron sulfide is a substance, because there is no method of purifying it, and a t best it is a dubious substance whose composition may depend upon the conditions of formation. Magnetic separation of a mixture is not a general method. The use of carbon disulfide for the separation is dangerous; for beginners in chemistry it is inexcusable. Used as a lecture demonstration, there are no striking color changes, and the product does not look very different from one of the reactants. Deiinite composition cannot be demonstrated without quantitative methods which liberate sulfur dioxide in the lecture room. The direct combination of antimouy and iodine in an inert solvent provides a more convincing lecture demonstration and student experiment. Iodine is exhibited. It is shown that iodine is soluble in an organic solvent such as tetrachloroethane, recommended because of its high boiling point, or carbon tetrachloride, which is less satisfactory because it increases the time of reaction. It may also be shown that iodine can be recrystallized from the solvent and is therefore to be regarded as a pure substance. It should be demonstrated that no change takes place when a solution of iodine in the solvent is heated. Antimonymetal is then exhibited in both the powdered and crystalline forms. A piece of antimony may be melted in a crucible to show that it has a definite melting point; that it will solidify to the original antimony and is therefore a substance. It is also shown that antimony powder is insoluble in the solvent. Iodine is then ground with an excess of powdered
antimony. The mixture is examined, stressing the fact that sufficient grinding will make it appear homogeneous. The mixture is then separated by pouring a few milliliters of the solvent into the mortar, mixing, and then transferring to a filter paper. The residue of antimony is washed with small amounts of the solvent uutil substantially free from iodine. The recovered antimony may be roughly dried on filter paper. Then i t is pointed out that iodine crystals may be recovered from the solution by evaporation of the solvent, actually demonstrated in case carbon tetrachloride is used. Following this, the antimony is scraped from the paper and returned to the iodine solution. The mixture is heated uutil the iodine color disappears, leaving a yellow liquid which contains suspended antimony. If desirable a t this point, iodine solution may be run in from a buret to show that the antimony requires a 6xed amount of iodine; however, this is time-consuming and may involve large volumes. When all the iodine has reacted, the hot solution is filtered or decanted into a clean container. In a few minutes, bright red crystals of antimony triiodide appear. The difference in appearance from either of the starting substances is striking. It may then be pointed out that this also is a substance because i t can be purified by recrystallization. The crystal suspension is warmed until a clear yellow solution results. Upon cooling, red crystals of the triiodide again appear. The product may be removed by filtration a t room temperature. It may also be stated a t this time that the product has a definite melting point, different from the melting points of either of the starting substances. The advantages of this over the iron-sulfur experiment are obvious. The fireproof solvent is important from the safety viewpoint; but i t should be remembered that the vapors, although non-flammable, are injurious if inhaled in large amounts.
