The entropy of a substance increases with its molecular weight and complexity and with temperature. The entropy also increases as the pressure or concentration becomes smaller.

The entropy of various parts of the system may change, but the total change is zero. Furthermore, the system does not affect the entropy of its surroundings, since heat transfer between them does not occur. Thus the reversible process changes neither the total entropy of the system nor the entropy of its surroundings.

Several factors affect the amount of entropy in a system. If you increase temperature, you increase entropy. (1) More energy put into a system excites the molecules and the amount of random activity. (2) As a gas expands in a system, entropy increases.

On the other hand, the change in volume of a liquid is appreciably low upon small increases in pressure that should substantially compress a gas, so the change in pressure of a liquid makes a smaller negative contribution to the change in entropy.

Changes in volume will lead to changes in entropy. The larger the volume the more ways there are to distribute the molecules in that volume; the more ways there are to distribute the molecules (energy), the higher the entropy. An increase in volume will increase the entropy.

As well, increasing the volume of a substance increases the number of positions where each molecule could be, which increases the number of microstates. Therefore, any change that results in a higher temperature, more molecules, or a larger volume yields an increase in entropy.

For a certain number of atoms or molecules, an increase in volume results in a decrease in concentration. Therefore, the entropy of a system increases as the concentrations of the components decrease.

(2) The mass of the atoms and molecules which are moving—the greater the mass, the larger the entropy.

Since entropy is a state function, the entropy change of the system for an irreversible path is the same as for a reversible path between the same two states. In classical thermodynamics, the entropy of a system is defined only if it is in thermodynamic equilibrium.

Dissolution of a solute normally increases the entropy by spreading the solute molecules (and the thermal energy they contain) through the larger volume of the solvent.

Solutions have a greater entropy than pure liquids as the particles in a solution are more separated and solvent molecules separate the solute particles. If the number of moles of gas increases in a chemical reaction (as shown by the stoichiometry of the reaction) then the entropy also increases.

When a liquid crystallizes into a solid, it gives off heat to its surroundings (the latent heat of fusion). So even though the crystal has low entropy, its formation increase the entropy of the surroundings enough so that the process has a net positive entropy change for the universe.

Having said that, the entropy of a crystal is actually lower than the entropy of the same substance in solution. This isn’t paradoxical because, as others have pointed out, the crystallisation process releases heat, and this heat increases the temperature of the crystal’s surroundings through the usual dS=δQ/T.