Question

A chemist has a substance which was either phenol or resorcinol. He puts the substance in a test tube, and puts the test tube in a beakar of boiling water. The substance melted. what was the substance and could you say whether the substance was pure or not

Answer 1

Hi,

The melting point of phenol is 40.5 degrees centigrade while that of resorcinol is 110 degrees centigrade.

So, the substance is definetely phenol.

Yes melting point can be used to determine purity of a substance.

Melting Point Determination

A melting point can be used to identify a substance and to get an indication of its purity. The melting point (or freezing point) of a solid is the temperature at which the solid exists in equilibriumwith its liquid state under an external pressure of one atmosphere. Both the melting point range (the interval between the beginning of liquefaction and complete liquefaction) and the temperature of complete liquefaction are valuable indicators of the purity of the solid compound. A pure crystalline organic compound usually possesses a sharp melting point and it melts completely over a narrow temperature range of not more than 0.5-1.0^oC, provided good technique is followed. The presence of even small amounts of impurities usually produces a depression of the temperature at which melting is complete and usually produces a marked increase in the width of the melting point range. For example, if a sharp-melting unknown substance X is suspected of being identical with some known substance A, the two should have the same melting points. If A is reported to have a melting point rather different from that observed for X, the two substances may be identical (the small differences being due to variations in technique of determining the melting points). Whether they are indeed identical can often be deduced quickly if a sample of A is available, by determining a mixture melting point. A mixture of X and A should have the same melting point as that of either substance alone, provided the two substances are identical.

If X and A are not the same substance (even though they separately have the same melting point), then a mixture of the two will usually show a lower melting point and a broader melting point range than either substance alone. This is because each substance acts as an impurity in the other. Miscible or partially miscible impurities, even when present in small amounts, usually lower the melting point and broaden its range.

A wide melting point range usually indicates that a substance is impure, but it may also result from the fact that the pure substance undergoes some decomposition prior to reaching its melting point. In some cases, the material undergoes a slight liquefaction and contraction at a temperature below the true melting point; in others, the material may decompose and discolour so badly that a definite melting point cannot be observed.