Answer:The yellow crystalline substance is a compound. The green gas and red powder are elements.
Step-by-step explanation
Elements and compounds are considered pure substances. Elements are made up of a single type of atom while compounds are composed of two or more elements. One example of a compound is the table salt (NaCl) which is made up of one atom of sodium and one atom of chlorine. The elements in the compound are combined chemically and there is a definite ratio of these elements. They are formed by a chemical reaction where chemical bonds are formed. Furthermore, the components of the compound can be separated through several means such as extraction, distillation, etc. The process of heating the yellow crystalline substance resulted in the separation of its components namely, the green gas and red powder.