Which statement defines the enthalpy of hydration? the change in enthalpy when 1 mol of a gaseous ion becomes fully hydrated
the change in enthalpy when gas ions condense to form 1 mol of an ionic solid
the change in enthalpy as heat is lost or gained in a reaction
the change in enthalpy as temperature rises in a liquid

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

increased stability in the completion of fluorine's outer shell.