Which of the following molecules may show a pure rotational microwave absorption spectrum: (i) H2, (ii) HCl, (iii) CH4, (iv) CH3Cl, (v) CH2Cl2?

Explanation:

H₂ , HCl , CH₃Cl , CH₂Cl₂   Yes

CH₄    No .

Symmetric molecules do not have rotational microwave absorption spectrum because they do not possess permanent dipole moment so they lack polarisability .

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.