What is the proportion of oxygen in the air?

If you remember anything from your physics and chemistry lessons, you will recall that among the gases that make up the atmosphere and the air we breathe is oxygen (O2). It is often said that oxygen is necessary for life, but in fact, it is dioxygen. So what is this molecule that is involved in every breath? Let's go back to what we all learned in school a few years ago… and pick up the pieces.

Oxygen, a gas necessary for life and respiration

Useful reminders about dioxygen

Most of us are aware that when we breathe, the gases that leave our bodies are different from those that enter. The carbon dioxide that we emit is well known and is one of the main greenhouse gases (even if it is not through breathing that humans emit the most carbon dioxide).
Let's go back to chemistry basics. Dioxygen is a colourless and odourless gas that is composed of a molecule made up of two oxygen atoms (O2).
We also know that through photosynthesis, plants (plants, trees, leaves, and other plant species that are mainly green in colour) produce these oxygen molecules.
You may not know all its characteristics, but dioxygen is all around you… and even inside you!

The oxygen atom, never alone!

When talking about oxygen in the atmosphere or breathing, this is a misnomer. We are talking about dioxygen. It is thus this gas that we breathe because it is in this form that oxygen is found in the air.
In fact, to better understand this information (which we have all seen in class but which many of us have forgotten), you need to know that although the oxygen atom is indeed very present on Earth, it is never alone. Oxygen atoms are always found in the atmosphere together with other atoms forming molecules.
Thus, in the air (essential for breathing), we find the oxygen O2, the carbon dioxide CO2 and the ozone 03. But in water, it is different because the water molecule (H2O) is an alloy containing 1 oxygen atom associated with 2 other hydrogen atoms.
Note that dioxygen is also the necessary oxidizer for all combustion: in fact, dioxygen is the only gas present in the atmosphere capable of allowing combustion.
Well, that's it for the reminders of what some of us learned many years ago!

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Why is dioxygen essential for breathing (and for life)?

Oxygen is an essential gas for the proper functioning of the human body (among others) because our cells consume it continuously and even more so during physical effort. The body therefore unconsciously and automatically seeks oxygen from the atmosphere through breathing.
Indeed, oxygen was described as a "vital gas" by Robert Boyle in 1658 because it is taken from the air with each breath and supplies each of our organs at all times. Without oxygen for a prolonged period (e.g. underwater), asphyxiation and death are the results.
For millions of years, living things on Earth have used oxygen molecules to live and grow. For millions of years, plants have also been responsible for the production and release of oxygen into the atmosphere.

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How much dioxygen is in the air?

How to detect the presence of carbon dioxide and oxygen?

Let's see who pay attention in school years ago when it was necessary to detect the presence of oxygen in the atmosphere! To identify the presence of the gases carbon dioxide and dioxygen in the air, you need limewater… and glowing charcoal.
If you recall the experiment, limewater clouds in the presence of carbon dioxide while the presence of the oxygen molecule can be identified by a piece of glowing charcoal.
But let's turn to the main topic of this article: the amount of dioxygen in the air.

The share of dioxygen among the different gases in the atmosphere

As we have seen, the air we breathe is a mixture of several gases. By volume, 21% is oxygen (20.95% to be exact), 78% is nitrogen and 1% is other gases such as water vapour, ozone, methane, and carbon dioxide. In terms of volume, the proportion of oxygen in the air is therefore a minority. However, it is the presence of this gas in the atmosphere that allows respiration and therefore life.
But this was not always the case. Dioxygen has been abundant in the atmosphere only since the Great Oxidation Event that occurred about 2.4 billion years ago.

What we still call "oxygen" today and which is referred to as the gas necessary for human life is therefore not oxygen, but dioxygen, a combination of two oxygen atoms (O2). Although a minority in volume in the atmosphere, it is vital for breathing. Fortunately, we have plants to guarantee production!