Helium gas is classified as a pure substance. A pure substance is made up of only one type of particle. In the case of helium, it consists entirely of helium atoms, which are not mixed with any other elements or compounds.
To prove this, we can consider the characteristics of a pure substance. Helium has a consistent composition throughout; its properties, such as boiling point and density, remain the same regardless of the sample size. Unlike mixtures, which can have varying properties depending on their components, helium exhibits uniform properties because it only contains helium atoms.
Furthermore, helium is a noble gas, meaning it is chemically inert and does not readily combine with other elements. This stability further confirms that helium is not a mixture, as it does not form compounds with other substances unless under extreme conditions.
In conclusion, since helium gas consists solely of helium atoms and does not contain any other substances, it is clearly a pure substance.