A student heated a mixture of iron filings and sulfur strongly. He saw a red glow spread through the mixture as the reaction continued. At the end of the experiment a black solid had been formed.

a. Explain what the red glow indicates.

b. Give the chemical name of the black solid.

c. Write a word equation and a balanced chemical equation to represent the reaction which has taken place.

d. The black solid is a compound. Explain the difference between the mixture of sulfur and iron and the compound formed by the chemical reaction between them.