Compounds used in an experiment should be reasonably pure to ensure accuracy of information and the true understanding of its nature. In this experiment, melting point is established as one of many methods to indicate whether a substance is pure and to identify an unknown material. The experiment focuses on solids although there are additional approaches to finding the purity of liquids. These two solids are Naphthalene and Biphenyl. Their literature melting point values are:
Naphthalene 80.26° C
Biphenyl 69-71° C
The capillary technique and an electric Mel Temp device were used in determining the MPs. The results found that the solids were indeed pure. When deducting an experiment finding the identity of an unknown, it is learned that binary mixtures have different melting behaviors. A eutectic point is verified as the lowest point at which the mixture melts1. Group 9 and 10 was given Acetanilide and 2-Cloroacetamide with an unknown. The unknown substance is mixed 50/50 with each of the known substances. The unknown was determined through an un-depressed MP. It was concluded that the unknown substance had the matching melting point of Acetanilide at 113-115°C2.
The purity of a substance can be revealed through the technique of observing its melting point. Furthermore, this method can aid in discovering an unknown solid subjected to a mixed melting point. Generally, a pure solid will melt sharply with a range of less than 1-2°C whereas an impure solid would have a broad range greater than so1. The purpose of this experiment was to compare the sharp and depressed melting point ranges of several ratios of Naphthalene and Biphenyl mixtures to its pure compounds. The skills obtained through the observation of MP ranges with the capillary technique allowed for a successful trial in the ability of determining an unknown solid based on its mixed melting point. If two solids are the same substance, one would notice a MP that is rapid from it’s sintering point to being completely melted. The MP is said to be sharp/un-depressed and that of the opposite for an impure or binary mixture. The MP would be broad/depressed, as the sintering point to the complete melted state would vary in time and degree1.
Materials and Method
100% Naphthalene (N)
100% Biphenyl (B)
90% B, 10% N
90% N, 10% B
70% N, 30% B
70% B, 30% N
50% N, 50% B
100 g Acetanilide
100 g 2-Cloroacetemide
100 g Unknown Solid
Melt-Temp Apparatus (group 10 obtained a digital device)
Finding the Eutectic Point:
Pure Naphthalene and Biphenyl was scooped onto a spot plate and crushed with a glass rod so the solid crystals would be fine enough for the glass capillaries.
A height of ~2 mm of pure Naphthalene and Biphenyl were then loaded into the capillaries. Each capillary was inserted into the Melt-temp device to be heated until melted. The ranges noted and reported were the degrees after which it begins to soften and shrink (“sintering”)1. The lowest temperature from when actual liquid is visible in the capillary is the first marked melting point. The end of the range is the temperature at which the substance is completely melted and an apparent meniscus has formed.
The Melt- Temp apparatus was first heated to temperatures near each pure substance’s literature MP value. Without being said, the device cooled to a low enough temperature before each trial. The process is repeated for all 6 substances and two trials were conducted per substance to ensure accuracy and concrete MPs. All the mole percent compounds of N & B were pre-measured. Group 9 and 10 was given the mole percent and mixture of 90% B, 10% N.
Identification of an Unknown Using Mixture Melting Points:
Unknown ‘E’ was issued to group 9 and 10. The MP of unknown ‘e’ was determined and logged using the techniques from the first experiment…