Laboratory > Melting Point Determinations
The purpose of experiment is melting point determinations of any substances and effect of the impurities on the melting point.
First we will select the a known substances and determine its melting point.Then we will repeat the melting point determination of the knownsubstance until good results are obtained.We will contaminate the known substance with a foreign substance.And then we will determine its melting point and observe the effect that contamination has on the sharpness of a melting point.Finally we will obtain an unknown substance and determine its melting point and we will record the results.
1. There are four aparatus used to measure the melting points of organic substances.They are melting point bath,long-necked flask,thiele tube and electrically heated melting point.
Ethyl alcohol,acetone,sugar and salt are impurities substance.A soluble impurity contributes to the total vapour pressure,thereforelowering the partial vapour pressure required of the pure substance in the melt and thus lowers the temperature necessary for melting.Addition of more impurity will produce corresponding lowering in the partial vapour pressure of the pure substance and hence ,lowering of the melting point.
Determination of the temperature at which the solid and liquid phases of a substance are in equilibrium is tedious and time consuming; it is also quite difficult with a small amount of sample. Thus, in practice, most melting points are determined as capillary melting points, which can be done quickly with a small amount of sample. A capillary melting point is defined as the temperature range over which a small amount of solid in a thin walled capillary tube first visibly softens (first drop of liquid) and then completely liquefies. Melting points recorded in the chemical journals are capillary melting points unless otherwise stated.
A solid is said to melt sharplyif the melting point range is 0.5 – 1.0 deg. C. A pure solid will generally melt sharply because the forces of attraction between its particles are the same. However, the presence of a foreign particle in a crystal lattice interrupts its uniform structure and the forces of attraction are weakened.
An impure solid melts at a lower temperature and over a wider range. Thus, a solid’s melting point is useful not only as an aid in identification but also as an indication of purity.
We found the melting point of the naphtallin was 82 deg. C.and the range of the naphtallin was 82-86 deg.C.Then we did the experiment with unknown substances and the value of the its melting point was 188 deg.C.We found its melting range 188-192 deg. C.And then we predicted that the substances was succinic acid.Because its melting range was 187-192 deg.C.Our results was a little different from these values.These differences may be source of the experimental error such that we may read the thermometer wrong.
1-We use the capillary tube for determining the melting point of the unknown substances.We put the unknown substances in the capillary tube and then we hold the capillary inside the thiele tube.Then we can determine the melting point.The capillary tube must have thin wall and small diameter so temperature is the same value capillary and thiele tube.And we determine the correct point.
2-The instrument is superior to hot stage for rapid work,since the inner tube and thermometer can be quickly transfered to a cold.Thiele tube for subsequent determinations,where as a hot stage is so throughly insulated as to retain its heat for a long period of time.This is the advantages of the this method.Another advantage is correct result is obtained with using thiele tube but experiment with this method take very long time.This is the disadvantages of the method.