The addition of perchloric acid to pure water causes the conductivity to

Answer is: increase.

Balanced chemical reaction of dissociation of perchloric acid in water:

HClO₄(aq) → H⁺(aq) + ClO₄⁻(aq).

Ionic compounds (in this example perchloric acid) conduct electricity, because it dissociate in ions and ions conduct electicity.

Ionic substances dissociate in water on cations and anions.

Perchloric acid (HCO₄) is one of strongest inorganic acids.

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 it's c. i'm not sure but i think so.