is oxygen a mixture or pure substance

Is oxygen a mixture or pure substance? It can be pretty confusing because we all know oxygen is essential for our survival, but we actually know very little about it.

You may already know that oxygen can dissolve in water, supports combustion, and obviously plays a big role in life on earth.

But, when talking about its chemical makeup, you may still be looking for the answer to, “Is oxygen a pure substance or mixture?”

Yes, oxygen is a pure substance because it contains only one type of molecule. 

The Wonderful Oxygen

Oxygen is an element made up of two atoms to create a chemical compound. It is written as O2 in formulae.

symbol of oxygen

When you talk about oxygen, you will also find people mentioning ozone. Ozone, is also oxygen and is found very high up in the atmosphere and has three atoms, so ozone is written as O3.

Fact: Swedish apothecary Carl Wilhelm Scheele discovered oxygen in 1771 but did not release the findings until 1774. 

No Life without Oxygen

Of all the gases, it is oxygen that gives us life.

Not only do we breathe it in, but almost all the processes at work in our bodies and the bodies of other life forms need oxygen to function.

It is an essential gas. Without it, there would be no life on earth.

oxygen from the trees

In fact, more than half of Earth’s crust is composed of this material, making it the most common substance on the planet.

Moreover, oxygen makes up about 21% of Earth’s atmosphere, and it is the most prevalent element in the human body, composing about 65% of its mass. 

Is Oxygen a Mixture or Pure Substance?

On its own, oxygen is a pure substance.

In its natural state, like all gases, oxygen molecules are free to spread out to occupy the space they are given.

molecules of oxygen

Interestingly, oxygen was not present on early Earth.

It was only added to the atmosphere after simple single-cell organisms in shallow oceans began to photosynthesize sugars for food using energy provided by the sun.

It took many millennia for enough oxygen in the atmosphere to support life as we know it. 

Fact: Oxygen is crucial to all forms of life on Earth, mainly through the oxygen cycle, which is the transfer of oxygen between the atmosphere and living things. 

A Friendly Molecule

Oxygen is a friendly molecule. Scientists describe it as being ‘very reactive’.

This simply means that oxygen is able to combine with the atoms and molecules of many other chemicals.

Knowing just a few could prompt someone to ask, is oxygen a pure substance or a mixture?

scuba diving

For example, in respiration, when we breathe in O2, it combines with carbon, a waste product our bodies produce and is expelled as CO2 when we breathe out.

Plants perform this process the other way around. They take in CO2 and expel O2. This makes both processes vital for maintaining life on the planet. 

How Do You Identify a Pure Substance?

As mentioned already, a pure substance will only have one kind of particle, which is true in the case of oxygen.

At the same time, the substance will have only one type of molecule or atom.

And because of this, you can call all the elements as pure substances, as they have only one type of particle.

oxygen from the water

It is important to understand that you just cannot separate a pure substance into another type of matter using any physical process.

It comes with a fixed boiling and melting point as well. With these points in mind, it is easy to categorize oxygen as a pure substance. 

Oxygen as a Pure Substance

The element oxygen is everywhere. It’s the third most abundant in the universe. The other two being hydrogen and carbon.

On its own as O2, oxygen is a pure colorless gas that is all around us as part of the mix that is air.

One could be forgiven for thinking that oxygen doesn’t do that much. This couldn’t be further from the truth.

oxygen cylinder tank

It is, in fact, the most reactive of the non-metallic elements, and has many characteristics:

  • Oxygen is very reactive, and how reactive is measured as valency.
  • Oxygen is easily able to form compounds with many other chemicals.
  • The result of any oxygen-based reaction is an oxide.
  • Oxides exist in the environment forming minerals, amino acids and proteins.
  • Oxides can be acid, basic or neutral.
  • Amphoteric oxides can be either acid or base depending on the conditions.
Fact: Photosynthesis is the primary source of atmospheric oxygen, as without plants, the air would contain significantly less of this gas. 

How Oxygen As a Pure Substance Reacts with Other Elements?

The group of compounds that form when oxygen from air or water reacts with other elements are called oxides. They are usually solids or gases.

oxygen as a pure substance

Water is an oxide, specifically hydrogen oxide. It is a compound of two hydrogen molecules and one oxygen molecule, H2O.

Other common examples of oxides are the sulfur oxides SO2 and SO3, carbon monoxide CO and nitric oxides NO and NO2 and iron oxide Fe2O3

Oxides in the Environment

We can see the effects of oxygen when combined with other environmental elements.

One reaction, iron and oxygen combining to form iron oxide is especially prevalent. We all know it as rust.

a car full of rust

Even as the earth began to take shape, the rocks and soils of Australia were rich in iron displaced from the earth’s core by violent tectonic motion.

Once open to the air and oxygen, the rock and soil took on the distinctive rust color of iron oxide. 

Realizing this prompted scientists to hypothesize that the red planet Mars must have had oxygen at some point in its history since its soils are rusty red too. 

Reactivity of Oxygen as a Pure Substance and Its Valency

Oxygen finds it so easy to combine with other chemical elements that there are many examples of oxides although they have different properties.

Because oxygen is so friendly and able to combine with most of the chemicals on the elementary table, scientists have had to find ways to sort and classify them.

scientist conducting a research

One way oxides are sorted is by their valency. Valency is a measure of how easily the molecules combine.

Using this measure, oxides fall into two groups, simple oxides, and mixed oxides.

  • Simple oxides are made up of one metal or semimetal and one oxygen.
  • Mixed oxides are formed when simple oxides are mixed.

Red lead is a mixed oxide. It is a combination of lead dioxide and lead monoxide.

The formula recording the reaction that makes red lead would be – PbO2 + PbO    Pb2O3

Metallic Oxides or the Minerals

As the name suggests these are oxides that are combinations of oxygen and metals.

They are mostly found as naturally occurring minerals. The red lead would fall into this category too.

different kinds of minerals

Other examples are MgO4, magnesium oxide, CaO or oxocalcium, known to most as quick lime and BaO, barium oxide which is used in making cathode ray tubes, crown glass and solvent dyes. Its reaction formula would be,

2Ba+O2     2BaO 

What are Some Pure Substances Besides Oxygen?

When you talk about pure substances, it means you are referring mostly to elements.

chemical elements

In most cases, elements are pure substances, and some good examples include:

  • Oxygen
  • Copper
  • Gold
  • Diamond
  • Chlorine

Interestingly, many compounds are also categorized as pure substances, such as:

  • Baking soda
  • Crystals
  • Salt
  • water 

Chemical Properties of Oxygen as a Pure Substance

As a pure substance, oxygen has many interesting chemical properties, which make it usable in many different sectors.

For instance:

  • Oxygen cannot burn itself but you cannot burn anything without oxygen.
  • Oxygen is present in different compounds, including carbon dioxide, water, etc.
  • Oxygen units with different substances and initiates a process called oxidation.

Other than these, it is interesting to see how the action of UV radiation on oxygen leads to more oxygen production. The electric discharge of oxygen can also lead to the production of oxygen.

Fact: Too much oxygen is hazardous for us and can lead to a condition known as the bends, which is especially dangerous for scuba divers and astronauts. 


Is oxygen a mixture or pure substance? By definition, any element that contains only one molecule or single type of particle is considered pure.

And the same is the case with oxygen, as it contains only one type of molecule.

As a pure substance, it has many properties, like it is a tasteless, odorless, and colorless gas with a density higher than air. And there are many other characteristics making oxygen special in so many ways.