Classification of Chemical Substance
The aim of this experiment is to investigate the properties of several substances with the purpose of determining whether they are molecular, ionic, macromolecular or metallic. Introduction Chemical substances could be named ionic, molecular or metallic substance, based on the kind of bond present in it.
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Solid ionic compound is composed of ions, which are held together by large electrostatic forces. Bonding in molecular substance is caused by the sharing of electrons by atoms. When the stable aggregates resulting from covalent bonding contain relatively small numbers of atoms, they are called molecules, if aggregates are very large and include essentially all the atoms in a macroscopic particle, it is called macromolecular. Bonding in metallic substances is caused by the free movement of electrons. Materials
Test tubes, Bunsen burner, distilled water, thermometer, portable ohmmeter, penny, rubber washer, pliers Procedure a. Melting Point A pea-size sample of a substance is taken. If the sample melts between 100° and 300° C, it will take more than gentle warming, but will melt before the test tube turns the flame of Bunsen burner to yellow-orange color. Above 300° C, there will be increasing color; up to about 500° C one can still use a test tube and a strong burner flame; however in this experiment there will not be measured any m. p. which are above 500oC.
Sample is heated, with a loosely closed stopper. During the heating process, sample is observed in order to reveal decomposition, sublimation, or evolution of water. b. Solubility and Conductance of Solutions Sample is placed in a regular size test-tube with 2 mL of solvent and stirred. It can dissolve immediately, partially, remain the same or produce a cloudy suspension. Solubility tests with distilled water and the two organic solvents are made and results are recorded. Conductance measurements will be made on water solutions by means of portable ohm meter (measures electrical resistance).
A solution with a high resistance has a low electrical conductance, and vice versa. Solutions which have a low resistance, 1000 or less are good conductors. Distilled water has a high resistance; but impurities and solvent, can decrease it. Firstly electrodes are rinsed with distilled water, and then resistance is measured for the sample solutions. For purposes of this experiment, a solution with a resistance less than about 2000O is a good conductor, denoted “ G”. Between 2000 and 20,000O it is a weak conductor, denoted “ W”.
Above 20,000O we will consider it to be essentially non-conducting, and denote it with an “N”. a. Electrical Conductance of Solids and Melts Sample is put on the lab bench and touched with the two wires on the ohmmeter probe. Generally metals have a very low resistance; in powder form they even appear to have essentially infinite resistance. However, under pressure, metal powders show good conductance. Penny is put on the lab bench, small rubber washer with a hole, where powder is located, is put on the penny and covered with another one. Then it all is put in the jaws of insulated pliers.
Ohmmeter electrodes are put to each of the pennies, and pliers are squeezed. Pennies should not touch each other, and if resistance will gradually fall, it indicates that powder is metal. Another sample is put in the test tube and melted. Electrodes are heated for a few minutes with Bunsen burner and then applied to the melt. It is gently heated to ensure that no solid is crystallized on the electrodes. Some melts are good conductors. After the procedure, electrodes should be rinsed with distilled water, then two unknown are taken for further characterization.
Discussion After having performed the experimental procedure, results obtained were used to determine whether the observed substances were ionic, molecular, macromolecular or metallic. The fact that all the substances were soluble in water, revealed the fact that they cannot be macromolecular or metallic. Due to some technical problems, it was not possible to measure the melting point of the substances, so the data was taken from teaching assistants for the three substances. However, information about the m. . of two unknowns was missing.
This fact influenced the accuracy of determination the nature of these substances. Conclusion After having performed this experiment, it was clear, that the procedure used is very useful in determining the nature of the observed substances, as based on the melting point, solubility and electric conductivity, it is possible to find out whether the observed substance is ionic, molecular, macromolecular or metallic by means of comparing the properties of these groups accordingly.