When 0.60 mol NH3 is decomposed in a 1 Liter flask at 850 K, the equilibrium concentration of NH3 is measured as 0.12 M. Given that ammonia decomposes according to the reaction 2NH3(g) ⇌ N2 (g) + 3H2 (g). Required:
What is Kc for the reaction?

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.

i think the answer is terrae but not 100 percent sure. ▓▬▬ §