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

(a) The stability of crystal depends upon the magnitude of force of interaction in the consituting particels. Greater the force of attraction present, more will be the stability of the crystal . For example, ionic solids such as NaCl, KCl etc. Have very high melting and boiling points while the molecular crystals such as naphthalene, iodine etc. have low values of melting and boiling points.
(b) The melting poins of different substances are : `{:("(i) Ice = 273 K","(ii) Ethyl alcohol = 155.7 K"),("(iii) Diethyl ether = 156.8 K","(iv) Methane = 90.5 K"):}`
The intermolecular forces in molecules of ice and ethyl alcohol are mainly hydrogen bonding. The magnitude is more in ice than in alcohol as is evident from the values of melting point.
The forces in the molecules of diethyl ether are dipolar foces while in methane, these are mainly the van der Waal's forces which is quite evident from its melting point (least among the compound listed).