Spontaneous changes are ones in which the free energy of a system decreases. Heat energy is also called enthalpy. When heat is released, the change in the enthalpy for the system that is releasing the heat decreases, whereas when heat is absorbed, the change in the enthalpy increases. While a decrease in the enthalpy makes a process more spontaneous (favorable), the change in enthalpy alone cannot be used to predict whether an overall change is spontaneous. There is another factor that must be considered and that is the entropy. Entropy is a measure of disorder; when a system become more disordered, the change in entropy is positive. When a change entropy is positive, it makes the change more spontaneous (favorable). Nature loves disorder. But like enthalpy, changes in entropy alone cannot be used to predict whether an overall change is spontaneous. For that you need to determine the change in the free energy. The free energy change combines the enthalpy change and the entropy change together, along with the temperature, to produce a quantity that can be used to determine if a process is spontaneous or not. This is summarized in the following equation:

where is the change in the free energy, is the change in the enthalpy, is the change in the entropy, and T is the absolute temperature in Kelvin. The following tables lay out the conditions for when a process is spontaneous and when it is not.

The conditions for , which make a process spontaneous or not

	Process is	Conditions
+	nonspontaneous	always
–	spontaneous	always

The conditions for and , which make a process spontaneous or not

		Process is	Conditions
+	–	nonspontaneous	always
–	+	spontaneous	always
–	–	nonspontaneous	|| < |T|
		but can be made spontaneous by	Lowering T
+	+	nonspontaneous	|| >|T|
		but can be made spontaneous by	Raising T

The "| |" brackets mean "the absolute value of". A good example of the last situation where both and are positive, is the heating of solid water (ice) to convert it first to a liquid and then to a gas. The is positive because heat is being absorbed. The is positive because the water becomes more disorder as it change from a solid to a liquid, and from a liquid to a gas (See Figure 1)

Figure 1: The the heating of water. When heat is added to a substance the change in enthalpy is positive. When water changes from a solid to a liquid (melting), or from a liquid to a gas (vaporization), the change in entropy is also positive. Below the melting point, the changes in enthalpy and entropy combine to produce a positive change in free energy for melting, so melting is nonspontaneous (unfavorable). Above the melting point, because of the raised temperature, the changes in enthalpy and entropy combine to produce a negative change in the free energy for melting, so melting is spontaneous (favorable). The same statements can be made about the free energy change for vaporization below and above the boiling point of water.