لخّصلي

Application of the phase rule to two component system shows that there are three state functions that must be specified in order to describe the system.It is this crystallization of both solids that produces the eutectic halt in the cooling curve.The phase diagram for a two components system represents the plot of temperature against the mole fraction of one component.For a pure liquid or melt, the rate of cooling is proportional to the temperature difference but for a solution or melt of two compounds, the cooling curve shows a change, in slope as the latent heat of crystallization of one component is released and reduce the rate of temperature decrease.In contrast to the behavior of a pure substance, the solution does not freeze completely at one temperature

In order to continue to crystallize this component the temperature must continue to be lowered till the eutectic temperature whereby at this temperature, the cooling curve does level off and the solution that remains solidifies abruptly.At b1 the change off slope occurs and the temperature continues to decrease till C1 whereby at this eutectic temperature (C1) the second component begins to crystallize together with the first and the temperature remains constant until solidification is complete at d (eutectic halt).The abrupt leveling off of the cooling curve is called eutectic halt.

Application of the phase rule to two component system shows that there are three state functions that must be specified in order to describe the system.

These variables are the temperature, pressure and concentration (X) of one of the components.

This implies a three dimensional phase diagram however, it is) common practices to consider the system as a condensed one with the total pressure as fixed thus we can have a two-dimensional phase diagram with concentration and temperature as variables.

Phase diagram are constructed from experiments in which mixtures are liquefied and allowed to cool and temperature as function of time determined.

For a pure liquid or melt, the rate of cooling is proportional to the temperature difference but for a solution or melt of two compounds, the cooling curve shows a change, in slope as the latent heat of crystallization of one component is released and reduce the rate of temperature decrease.In contrast to the behavior of a pure substance, the solution does not freeze completely at one temperature

In order to continue to crystallize this component the temperature must continue to be lowered till the eutectic temperature whereby at this temperature, the cooling curve does level off and the solution that remains solidifies abruptly.

The abrupt leveling off of the cooling curve is called eutectic halt.

Figure 1A shows the cooling curve of a pure liquid while figure 1B shows the cooling curve of a melt of two components.

At b¹ the change off slope occurs and the temperature continues to decrease till C¹ whereby at this eutectic temperature (C1) the second component begins to crystallize together with the first and the temperature remains constant until solidification is complete at d (eutectic halt).

It is this crystallization of both solids that produces the eutectic halt in the cooling curve.The phase diagram for a two components system represents the plot of temperature against the mole fraction of one component.

This is shown in figure 2 for two components A and B. Points a and b represent the freezing temperatures of a pure A and B. Curves a e and be represent the depression of freezing point of each component as a result of addition of the other. The two curves meet at the eutectic point e.
The two components to be studied in this experiment are to be one of the following pairs:

1 - Naphthalene - biphenyl

2- Naphthalenep nitrotoluene

3- Naphthalene naphthylamine

4- Naphthalene - p - toluidine

If the chemicals are available then the class would be distributed to study all the systems above, however the procedure would be illustrated for the system Naphthalene-p-nitrotoluene. Similar procedure should be done for other eveteme