First, let's review the concepts of mixtures and pure substance:
A substance is matter that has a uniform and definite composition. All samples of substances, sometimes called pure substances, have identical properties. Substances may be either elements or pure compounds. They may also be solid, liquid, or gas.
A mixture is a mixture of several substances and you can identify them as homogeneous (only contains one phase) or heterogeneous (contains more than a phase).
- Steel is a mixture because is formed of various elements that are metals and it's a homogeneous mixture because you only see one phase.
- Helium is a pure substance because is an element as you can see in the periodic table (Helium is the second element of the table).
- Carbon dioxide is a compound. A compound is the result of a reaction as an example and compounds are pure substances, so carbon dioxide is a pure substance.
- Tea is a mixture because water and other soluble compounds like sugar are mixed. In this case, is a homogeneous mixture because you only see one phase.
- Gold is an element like helium, you can find it in the periodic table of elements too. It is the 79th element, so is a pure substance.
- Olive oil is a substance pure because it is a compound and remember that compounds are pure substances too because it contains various elements that are combined chemically.