What is the oxidation number of cl− in the perchlorate ion, assuming the cl−o bonds are all single bonds?

The molecular formula for perchlorate ion is: ClO₄⁻

The oxidation number of oxygen, O = -2

Let 'x' be the oxidation number of Cl-. Now the net charge on the system is -1

i.e.,  Cl + 4(O) = -1

(x) + (4)(-2) = -1

x = -1 + 8 = +7

Therefore, the oxidation number of Cl- in perchlorate ion is +7

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.

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