Q.'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?

The melting point measures the thermal energy needed to overcome the forces holding particles in the lattice, so a high melting point reflects strong forces and a stable crystal. Typical data-book melting points: water $273$ K, ethyl alcohol $\approx156$ K, diethyl ether $\approx157$ K, methane $\approx91$ K. …

Step 1 – Why melting point reflects stability.

Melting requires enough thermal energy to break the ordered arrangement of particles. The stronger the intermolecular (or interionic) forces, the more energy (higher temperature) is needed to melt the solid. Hence a high melting point indicates strong binding forces and a more stable crystal.

Step 2 – Melting-point data (from a data book).

Step 3 – Interpret the intermolecular forces.

• Water — extensive, strong hydrogen bonding (two O–H donors, two lone pairs) $\Rightarrow$ highest melting point.
• Ethyl alcohol — hydrogen bonding through its single O–H, but fewer H-bonds than water. …

Link melting point to the energy needed to overcome lattice forces (so higher m.p. = more stable). Then rank the four molecules by force type: H-bo …

• Claiming melting point depends on molecular mass alone, ignoring hydrogen bonding.
• Ranking methane above the others because "it is a gas" (that is a consequence, not the cause). …