Entropy is defined as a thermodynamic quantity that represents the unavailability of a system's thermal energy for conversion into mechanical work. It can also be referred to as the degree of disorder or randomness in the system. Entropy is increased when the solid or liquid reactants react to produce gaseous products or when the mole number of products is greater than the mole number of reactants. The atoms and molecules in gaseous state or having more molecules in the same container or volume are arranged in much more 'random' manner thus having a greater entropy level. Applying this concept of entropy, one way you can change the entropy is by adding more removing molecules from the system. Adding more molecules to the system will increase entropy as the degree of disorder is greater and more random, whereas removing some molecules from the system will decrease the entropy level. Increasing or decreasing the pressure will also alter the entropy level; decreasing pressure will mean that the volume occupied by the reactants multiplies thus increasing entropy. Pressure can also be increased to decrease entropy.