Stability of a crystal is reflected in the magnitude of its melting points'. Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane from a data book. What can you say about the intermolecular forces between these molecules?

Explanation

For a crystal to melt, the attractive forces holding the constituent paticles in the crystal must be overcome. At the melting point of a crystal, thermal energies of the constiuent particles of the crystal overcome the attractice forces holding the particles, and in consequence the crystal structure collapses and it melts.
Therefore, a crystal in which the constituent particles are held by strong attractive forces requires more heat energy for its melting and hence, possesses a high melting point. The forces of attraction among the constituent particles in a crystal also determine the stability of the crystal because stronger forces of attraction favour the close-packing of the particle and which, in turn, increases the stability of the crystal. Therefore, a stable crystal must have a high melting point and we can say that a crystal's melting point reflects its stability.

(i) `H_(2)O` molecules are capable of forming H-bonds with one another. Hence, molecules in solid water experience intermolecular forces of attraction arising from H - bonds. In solid water experience dipole-dipole attraction and London forces.
(ii) `C_(2)H_(5)OH` molecules are polar and capable oof forming hydrogen bonds with each other. So, the molecules in solid ethanol experience intermolecular forces of attraction due to hydrogen bonding. In addition, the molecules in solid ethanol experience dipole-dipole attractions and London forces.
(iii) `(C_(2)H_(5))_(2)O` molecules are polar but they cannot form hydrogen bond. In solid `(C_(2)H_(5))_(2)O`, molecules experience dipole-dipole attractions and London forces.
(iv) In solid `CH_(4)`, molecules experience only London forces. Melting point of a solid depends on the strength of intermolecular forces holding the constituent particles in the solid. The stronger the forces of attraction in a solid, the higher the melting point it possesses. Strength of attractive forces due to hydrogen bonding is stronger than the dipole-dipole attractive forces, which in turn is stronger than London forces. Forces in solid `H_(2)O` due to hydrogen bonds are stronger than that in solid `C_(2)H_(5)OH` because the number of hydrogen bonds per molecule in solid `H_(2)O` is greater than that in solid `C_(2)H_(5)OH`. THus, the order of strength of intermolecular attractive forces in the given solids is `H_(2)OgtC_(2)H_(5)OHgt(C_(2)H_(5))_(2)OgtCH_(4)`. This result in the following order of melting points-
`H_(2)OgtC_(2)H_(5)OHgt(C_(2)H_(5))_(2)OgtCH_(4)`