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?

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Solution

Stability of a crystal is directly proportional to the magnitude of its melting points. Higher is the magnitude of forces holding the constituent particles together, higher will be the melting point and higher will be the stability. Thus ionic crystals ( $N a C l$ , $K N O_{3}$ etc.) have very high melting points and stable crystal lattices. On other hand, molecular solids (naphthalene, iodine etc.) have low melting points and low stability.
The melting points of solid water, ethyl alcohol, diethyl ether and methane are $273 K$ , $155.8 K$ , $156.8 K$ and $90.5 K$ respectively.
Solid water and ethyl alcohol have higher melting points due to presence of inter-molecular hydrogen bonds. Since the extent of hydrogen bonding in solid water is greater than the extent of hydrogen bonding in ethyl alcohol, the melting point of solid water is higher than the melting point of ethyl alcohol. Polar diethyl ether involves dipole-dipole interactions. Non polar methane involves weak van der Waals forces (London dispersion forces) which are weaker than dipole-dipole interactions. Hence, the melting point of methane is much lower than the melting point of diethyl ether.

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'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?

'Stability of a crystal is reflected in the magnitude of its melting point'. 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?

Q. Assertion :Stability of a crystal is reflected in the magnitude of its melting. Reason: The stability of a crystal depends upon the strength of the interparticle attractive force.

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