Final answer:
The correct answer is option b. Oxygen (O₂) is not a mixture but an element, which is a pure substance consisting of only one type of atom. Even when different states of oxygen are considered, they are all the same element and do not become a mixture. Mixtures can involve oxygen, but the oxygen itself remains an element.
Step-by-step explanation:
The statement 'Oxygen (O₂) is a mixture' is false. Oxygen is not a mixture but a substance and more specifically, an element. An element is a pure chemical substance consisting of a single type of atom, and in the case of oxygen, it is made up of molecules containing two oxygen atoms (O₂). This differs from a compound, like water (H₂O), which consists of hydrogen and oxygen atoms in a fixed ratio. It's also distinct from a mixture, such as soil, which contains small pieces of a variety of different materials and is considered a heterogeneous mixture.
Oxygen can exist in various states including as a gas (O₂(g)), which is the most common form we encounter in the Earth's atmosphere; as a liquid (O₂(l)), which is oxygen in a cold and condensed form; and as a solid (O₂(s)), where oxygen molecules are tightly packed together. For example, liquid oxygen is used in rocket fuel, and solid oxygen may be demonstrated in high-pressure laboratory experiments. In any of these states, it remains an element, not a mixture.
Mixtures, such as a combination of hydrogen gas (H₂) and oxygen gas (O₂), can have varying properties, such as explosiveness in certain proportions. Yet, even in these mixtures, the individual gases remain distinct substances and do not combine to form new chemical compounds unless a reaction is initiated.