Entropy Changes Accompanying Changes of Phase

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Entropy Changes accompanying changes of phase:

 A) From Solid Phase to Liquid Phase:

Consider a case when a solid changes into liquid state at its fusion point. The process requires absorption  of heat(heatof fussion)
    Consider melting of one mole of a substance reversibly at the fusion point T, at constant pressure. Let Del(H) be the molar heat of fusion. The entropy change of process, del(S) will then given by as shown bellow:



B) From Liquid Phase to Vapour Phase:

 Suppose one mole of a substance changes from liquid to vapour state reversibly at its boiling point Tb, under a constant pressure. If del(H) is the molar heat of vaporisation ,then the entropy change  accompanying the process will be given by as shown bellow:




Since del(H) of fusion and del(H ) of vaporisation are both positive, the processes of fusion and vaporisation are both accompanied by increase of entropy.

If we consider the change of state from vapour to liquid or from liquid to solid,  then del(H) of vaporisation  and del(H) of fusion will both be negative and hence the process of condensation of vapour or freezing of a liquid would be accompanied by decrease of entropy

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