Sublimation and Melting Point Determination

ABSTRACT

This experiment was performed to recognize and explain the behaviors of the chemicals when it under go sublimation or melting. Using impure benzoic acid as the reacting substance, we obtained and proved its true and pure form through sublimation and melting point determination using the group's product obtained from the former process mentioned as the basis of the data.

INTRODUCTION

This experiment aims to make the students capable of defining/explaining sublimation, melting point, melting point range and other terms associated to the topic of the experiment. Explain why chemists determine the melting point ranges of the chemicals used and draw the structures given/name the structure of the compounds used in the experiment.

The melting point of a substance is the temperature at which the material changes from a solid to a liquid state. Pure crystalline substances have a clear, sharply defined melting point. During the melting process, all of the energy added to a substance is consumed as heat of fusion, and the temperature remains constant. A pure substance melts at a precisely defined temperature, characteristic of every crystalline substance and dependent only on pressure (though the pressure dependency is generally considered insignificant). Determining the melting point is a simple and fast method used in many diverse areas of chemistry to obtain a first impression of the purity of a substance. This is because even small quantities of impurities change the melting point, or at least clearly enlarge its melting range. Melting point determinations are more than just a classroom exercise in the organic chemistry lab. The test is an important technique for gauging purity of organic and pharmaceutical compounds. The determination of melting points is one of the oldest identification and test methods for organic substances. The melting point is easy to measure, tabulate and classify. Extensive collections of tables give the exact values of many pure, inorganic and organic compounds. The melting point determination is a fast and cost effective technique, which remains a strong link to the vast pre-instrumental chemistry literature. In a dynamic melting point determination, where true equilibrium between solid and liquid phase is never achieved, the Melting Point Range---defined as the interval between the onset and clear points- is a valuable indicator of purity of a solid compound. On the other hand, Sublimation refers to the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase. It is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram. At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases, the transition from the solid to the gaseous state requires an intermediate liquid state. Note, however, that the pressure referred to here is the partial pressure of the substance, not the total (e.g. atmospheric) pressure of the entire system. So, all solids, which possess an appreciable vapor pressure at a certain temperature usually, can sublime in air (e.g. ice just below 0oC). For some substances, such as carbon and arsenic, sublimation is much easier than evaporation from the melt, because the pressure of their triple point is very high, and it is difficult to obtain them as liquids.

EXPERIMENTAL

A. Compounds Tested

- Impure Benzoic Acid

B. Procedure

1. Sublimation of Impure Benzoic Acid

Five grams of impure benzoic acid was put in an evaporating dish and was covered with a perforated filter paper, since the solution is very volatile. An inverted watch glass, weighed by one of the members, was placed on top. During heating, the tissue paper on top of the watch glass was kept moistened with water. And waited for about 10-15 minutes until most of the impure benzoic acid has been vaporized. Then After heating, the set-up has been cooled.