In a nuclear power plant, the nuclear reaction is kept from going critical by keeping the rate of reaction safe. how do the control rods figure into this?

the control rods absorb extra neutrons keeping them from causing to many uranium atoms to split to quickly.

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.