How do alkali metals differ from alkaline earth metals

Alkali metals and alkaline-earth metals are similar, but they are different in some ways as well. Alkaline-earth metals are slightly less reactive that alkali metals and more dense. Alkali metals have a very low density so they can be cut fairly easily. Both of these metals have a silver color.

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.