Physical Science:

Entropy is a mathematical concept explains some useful phenomena of thermodynamics. The change in entropy tells whether the process can occur naturally or not. A natural tendency of systems is to lose order. Hence, entropy measures the disorderness. When salt is dissolved in water it becomes more randomized and will not reconstitute itself in the crystalline form again by itself, unless we apply heat energy from outside to the system and evaporate all water. When a hot and a cold body are put in contact to each other, heat energy begins to flow from the hot body to the cold body until they reach a thermal equilibrium (same Temperature). Heat will never move back the other way. To move heat from a cold body to a hot body an external work must be done by an external source such as a heat pump. Entropy is a measure of the system’s thermal energy per unit temperature that is unavailable for useful work. The concept of entropy provides the direction of spontaneous change for many processes. It tells us which process is allowed and which is completely prohibited even if it cannot violate the fundamental law of the conservation of energy. A block of ice placed on a hot pan surely melts, while the pan gets cooler. Such a process is called irreversible because no slight change will cause the melted water to turn back into ice while the pan gets hotter. In contrast, a block of ice placed in an ice water bath will either thaw a little more or freeze a little more, depending on whether a small amount of heat is added to or taken out from the system. Such a process is reversible because only an infinitesimal amount of heat is needed to change its direction from progressive freezing to progressive thawing.