Final answer:
The group of substances that contains only compounds is C) CO₂, CaCO₃, NH₃. Compounds are composed of two or more elements chemically combined in a fixed proportion, and all items in this group meet that criteria.
Step-by-step explanation:
The student asked: Which group of substances contains only compounds? The groups given were A) C, O, CH₄ B) H, He, F C) CO₂, CaCO₃, NH₃ D) F, Fe, He. A compound is a substance that contains two or more elements chemically combined in a fixed proportion. An element is a substance that contains only one type of atom. A mixture contains two or more substances that are not chemically combined.
Group A contains carbon (C) and oxygen (O), which are elements, and methane (CH₄), which is a compound. Group B contains hydrogen (H), helium (He), and fluorine (F), all of which are elements. Group C contains carbon dioxide (CO₂), calcium carbonate (CaCO₃), and ammonia (NH₃), all of which are compounds. Group D contains fluorine (F), iron (Fe), and helium (He), with fluorine and helium being elements and iron being a metal.
Therefore, the correct answer is C) CO₂, CaCO₃, NH₃ because this group contains only compounds, with each of these containing two or more elements combined in a fixed ratio.