why is energy required for the melting or boiling process ?

Question

why is energy required for the melting or boiling process ?

Answer 1

Solids have something called a lattice structure- the pattern in which the molecules allign themselves. They keep vibrating about the fixed positions. But when they melt, they need to break out of this pattern held together so far by the attractive forces of all the molecules around it. To break out of this pattern, they need energy.

As a liquid, molecules can move about the whole liquid, bumping and butting into one another. They experience an attractive force from the molecules all around them. Since the attractive force is more or less equal in all directions, the forces neutralize.. This happens all through the liquid body, except at the surface of the liquid- ther are no molecules above the liquid surface to attract. So the net attractive force at the surface is downwards.

Now, for the liquid to boil- the liquid molecules to escape the downward attractive force at the surface- they need to speed up sufficiently and then reach the surface, so that when they break out of the surface, they are not pulled back. This "speeding up" needs extra energy.

Answer 2

Energy is required to break the forces that hold the molecules in a liquid together. In the gas state, after boiling, the molecules are farther apart than in the liquid state. The analogy is the same in converting a solid into liquid by melting.

Answer 3

In melting, the crystal structure must be broken. This requires energy.
In boiling, molecules go from close proximity to being far apart. It takes energy to overcome the attraction.

Answer 4

to overcome the forces of attraction between molecules