a. "Stability of a crystal is reflected is reflected in the magnitude of its melting points" Comment.
b. Melting points of some compounds are given below water `= 273 K`, ethyl alcohol `= 153.7 K`, diethyl ether `= 156.8 K`, methane `= 90.5 K`. What can you say about the intermolecular forces between the molecules of these compounds?

Explanation

The melting point of a crystal depends upon the magnitude of forces holding the constituent particles together, which determine the stability. Higher the melting point, greater are the forces holding the constituent particles together and hence greater is hte stability. For example , ionic crystals such as `NaCl, KNO_3`, etc. have very high melting points and are stable. On the other hand, molecular solids such as maphthalene, iodine, etc, are less stable because they have low values of melting points.
The melting points of some compounds are :
Water = 273 K, ethyl alcohol = 155.8 K, diethyl ether = 156.8 K and methane = 90.5 K.
The intermolecular forces in water and ethyl alcohol are mainly hydrogen bonding The higher melting point of water than ethyl alcohol indicates that the hydrogen bonding in water is stronger than in ethyl alcohol. Diethyl ether is a polar molecule and, therefore, the intermolecular forces in diethyl ether are dipole-dipole interactiosn. On the other hand, methane is a non-polar molecular and the only forces present in them are the weak van der Waals forces (London dispersion forces). These are weaker than dipole-dipole interaction and hence methane has very low melting point than diethyl ether.