Under which conditions will gases best dissolve in liquids?

Gases best dissolve in liquid under low temperature. This is because when you increase the temperature it will cause an increase in kinetic energy. An increase in kinetic energy means there would be an increase in molecular motion of gas particles. This would cause for gas particles to escape to the gas phase thus making it difficult for it to be dissolved. 

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.