Molecular systems tend to move spontaneously to a state of maximum randomness or disorder. Molecular randomness, or disorder, is called entropy and is denoted by the symbol S. As a state function, entropy change, ΔS, depends only on initial and final states. ΔS has a positive value when disorder increases and a negative value when disorder decreases. The following conditions usually result in an increase in entropy: a change of phase: solid→liquid→gas, an increase in the number of gas molecules, or a solid dissolving to form a solution. Although the sign of the entropy change can be predicted as described above, the actual value of ΔS∘ must be calculated from the absolute entropy values, S∘, of the reactants and products: ΔS∘=S∘(products)−S∘(reactants).