Entropy is a measure of the randomness or disorder of a system.

1. In the reaction 2H(g) → H2(g), two moles of gaseous hydrogen atoms combine to form one mole of gaseous hydrogen molecules. Since the number of gaseous particles decreases, the randomness of the system decreases, leading to a decrease in entropy (\Delta S < 0).
2. During the evaporation of water (liquid to gas), the substance changes into a highly disordered gaseous state, so entropy increases.
3. In the expansion of a gas at constant temperature, the volume increases, giving gas molecules more space to move around, which increases the randomness and hence the entropy.
4. During the sublimation of a solid to gas, a highly ordered solid structure breaks down into a highly disordered gas, leading to a large increase in entropy.