When a .22-caliber rifle is fired, the expanding gas from the burning gunpowder creates a pressure behind the bullet. This pressure causes the force that pushes the bullet through the barrel. Suppose the barrel has a length of 0.674 m and an opening whose radius is 2.68 x 10-3 m. A bullet (mass = 2.6 x 10-3 kg) has a speed of 376 m/s after passing through this barrel. Ignore friction and determine the average pressure of the expanding gas.

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.