The temperature change depends on the reactivity of the metal. The student's results are used to place copper, iron, magnesium and zinc in order of
their reactivity
Describe a method to find the position of an unknown metal in this reactivity series.
Your method should give valid results.

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.