During which of the following processes does the entropy of the system decrease? salt crystals dissolve in water air escapes from a hole in a balloon ice melts at room temperature liquid water boils methane gas and oxygen gas combust to form carbon dioxide gas and liquid water

Entropy of the system decreases when methane gas and oxygen gas  combust to form carbon dioxide and water.

Explanation:

Salt crystals when dissolved in water the bond between the molecules of crystals are broken down increasing the movement between the atoms hence entropy is increased.When air escapes from a hole in the balloon,the entropy increases as the air present inside the balloon occupies small volume as compared to outside. Ice when melts at room temperature its entropy increases because the molecule in the ice are closely packed hence less movement between the molecules.When water boils the kinetic energy increases or themovement between the atoms increases which increases the entropy.

Hence, the option (a), (b), (c), and (d) are not suitable to the statement                       that  "the entropy of the system decreases".

During combustion of methane gas there is release of carbon dioxide gas and water .Here, entropy decreases due to the formation of water which is due to overall decrease in the  kinetic energy of the system.

Anions are negative charged ions