Yes, it is a pure substance but can be mixed with other gases.
Helium is a pure substance
Helium gas is a pure substance
Helium gas is a pure substance
Helium is pure as it is on the periodic table.
Helium gas is a pure substance
Helium is an element (and therefore a pure substance) yes
Neither. Helium is not a mixture at all, it is a pure substance.
Neither. Helium is not a mixture at all, it is a pure substance.
No. Helium is an element, which makes it a pure substance.
Helium is an element which makes it a pure substance.
In theory it is a pure element. However, due the the cost of obtaining really pure helium, it is likely to contain trace amounts of other gases.
No, helium is a pure substance and an element to be more specific. It has a constant composition making it a pure substance and it is homoatomic (has only one type of atom) which makes it an element.