9. Explain why you can't write the formula of the ionic compound that forms between Fe
and S?

See explanation

Explanation:

According to Fajan's rules, the degree of covalent character of a bond depends on the relative polarizing power of ions.

Generally, ions become more polarizing when they are small and highly charged. Large negative ions are more polarizable than small ones.

A very small positive charge concentrated on a small area such as in Fe^2+ makes the ion highly polarizing. Also, a large negative ion such as S^2- is highly polarizable.

As a result of these FeS has considerable covalent character hence we can not simply write the formula of the ionic compound that forms between Fe and S?

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.