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In thermodynamics, the entropy of fusion is the increase in entropy when melting a solid substance. This is almost always positive since the degree of disorder increases in the transition from an organized crystalline solid to the disorganized structure of a liquid; the only known exception is helium. It is denoted as خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta S_{\text{fus}}} and normally expressed in joules per mole-kelvin, J/(mol·K).

A natural process such as a phase transition will occur when the associated change in the Gibbs free energy is negative.

خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta G_{\text{fus}} = \Delta H_{\text{fus}} - T \times \Delta S_{\text{fus}} < 0,}

where خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta H_\text{fus} } is the enthalpy of fusion. Since this is a thermodynamic equation, the symbol $T$ refers to the absolute thermodynamic temperature, measured in kelvins (K).

Equilibrium occurs when the temperature is equal to the melting point خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle T = T_f} so that

خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta G_{\text{fus}} = \Delta H_{\text{fus}} - T_f \times \Delta S_{\text{fus}} = 0,}

and the entropy of fusion is the heat of fusion divided by the melting point:

خطأ رياضيات (اعرض بصيغة MathML إن أمكن (تحت التجريب): رد غير صحيح ("Math extension cannot connect to Restbase.") من الخادم "https://wikimedia.org/api/rest_v1/":): {\displaystyle \Delta S_{\text{fus}} = \frac {\Delta H_{\text{fus}}} {T_f}}

 Helium

Helium-3 has a negative entropy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative entropy of fusion below 0.8 K. This means that, at appropriate constant pressures, these substances freeze with the addition of heat.

 انظر أيضاً

• Entropy of vaporization

 References

• Atkins, Peter; Jones, Loretta (2008), Chemical Principles: The Quest for Insight (4th ed.), W. H. Freeman and Company, p. 236, ISBN 978-0-7167-7355-9 
• Ott, J. Bevan; Boerio-Goates, Juliana (2000), Chemical Thermodynamics: Advanced Applications, Academic Press, ISBN 0-12-530985-6 

قالب:Thermodynamics-stub