Is #O_2# a pure substance or mixture? Chemistry Matter Mixtures 1 Answer anor277 Sep 28, 2016 Dioxygen gas is a pure substance. Explanation: #O_2#, for instance the oxygen you would use from a tank in oxy-acetylene welding, is a pure substance. The oxygen we breathe now, is part of a mixture in air; approx. 21% by volume. Answer link Related questions What are some examples of mixtures? How do compounds and mixtures differ? How can mixtures be classified? How do mixtures affect boiling point? What mixtures are heterogeneous? What mixtures can be separated by chromatography? What mixtures can be separated by paper chromatography? How does the melting point of the substances change in a mixture? What is an azeotrope? Why does an oil-and-vinegar salad dressing have two separate layers? See all questions in Mixtures Impact of this question 5703 views around the world You can reuse this answer Creative Commons License