Final answer:
A volume of gas containing only one element is indeed a pure substance since it consists only of that one type of atom. Pure substances can be either elements or compounds, distinguished by having a single set of properties throughout.
Step-by-step explanation:
The statement that 'A volume of gas containing only one element is a pure substance' is true. A pure substance consists of a single type of matter, which can be either an element or a compound. Elements are substances that are made up of one type of atom and cannot be broken down by chemical means into simpler substances. Compounds consist of two or more types of atoms bonded together. Examples include iron, gold for elements, and water, sodium chloride for compounds.
When we refer to mixtures, these are combinations of two or more pure substances that retain their individual properties and can be separated into their component substances. Contrastingly, compounds exhibit properties that are distinct from the elements that form them. The identification of pure substances can be verified using methods such as melting and boiling points, or chromatography, as pure substances will exhibit a sharp, single-point melting or boiling point, and chromatography will result in one substance if the sample is pure.