What do Cl-35 and Cl-36 have in common?

they have same atomic number as they are the same element & electrons

they both have the atomic (proton number) as they are the same element hence they also they have the same number of electrons. Therefore they have similar chemical properties, as the electrons determine the chemical property of an element.

Explanation: The numbers 35 and 37 are the mass numbers for the two isotopes of chlorine. They both have the same atomic number (proton number ) as they are the same

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.