Final answer:
The processes that result in an entropy decrease of the system are the chemical reaction forming NO₂ from NO and O₂ gases, and making ice cubes in the freezer, as these transitions involve increased order or decreased dispersal of molecules.
Step-by-step explanation:
The entropy of a system is a measure of its disorder or randomness. When considering entropy changes in various processes, the following apply:
Diluting hydrochloric acid results in increased entropy, because solute particles become more dispersed in the solvent.
Boiling water results in increased entropy, as the liquid water transitions to gaseous steam where molecules are much more dispersed and disorder increases.
The reaction 2 NO(g) + O₂(g) → 2 NO₂(g) results in decreased entropy. Although all species are gases, the total number of molecules is the same on both sides of the equation, but the formation of a more complex molecule (NO₂) from simpler molecules (NO and O₂) often suggests less randomness or disorder.
Making ice cubes in the freezer, where water goes from a liquid to a solid state, results in decreased entropy since the molecules in the solid state are more ordered.
Therefore, the processes that result in an entropy decrease of the system are the formation of NO₂ gas from NO and O₂ gas and making ice cubes in the freezer.