Consider the liquid phase reaction
a → bra = -kca
with k = 0.23 min-1
if the reaction is carried out in a plug flow reactor sketch the concentration of a as a function of volume, ca(v). given that the volumetric flow rate is 10 l/min determine the reactor volume necessary to reduce the exiting concentration of a to 10% of the entering concentration.
calculate the volume of a cstr operated at steady state for the conditions used to estimate the plug flow reactor volume in part (a).

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.