The air around us


A thick layer of air called atmosphere surrounds our Earth, reaching up to thousands of ki-lometres above the Earth´s surface. The composition of atmosphere depends on the height it reaches above the Earth´s  surface. The lowest layer of atmosphere, called troposphere, which is about 7 kilometres thick, and its composition are important for our life. The troposphere is the thickest part of the atmosphere. With the increasing height above the Earth´s surface, the density of the atmosphere decreases. The density of dry air at the Earth´s surface and the temperature of 20 °C is 1.2047kg∙m (-3) , which corresponds to the weight of 1.2 g per one litre of air. By the Earth´s gravitational pull, all atmospheric particles are constantly drawn down to the Earth´s surface making the whole atmosphere to stay with the Earth instead of drifting off into the space. Atmospheric pressure is then its product.

Atmospheric pressurek

Atmospheric pressure exerts on the whole Earth´s surface and on all earthbound objects, including us – human beings. If the atmospheric pressure decreases, gas-filled, flexible containers expand. If the atmospheric pressure increases, gas-filled, flexible containers are deformed and their volume decreases.We can demonstrate this effect by placing a small piece of a sweet like a marshmallow (Fig. 1) into a syringe. We close the syringe by the plunger, which we leave in the upper position and close the end of a syringe tip by a thumb. When pushing the plunger down, a pressure higher than the atmospheric one is created inside the syringe. Due to this, the marshmallow is deformed and its volume is decreased. The demonstration of low pressure is similar. We close the syringe by the plunger that we push down until it just reaches the marshmallow but not any further to avoid its deformation. We close the end of a syringe tip by a thumb again. When pulling the plunger up a low pressure is created inside the syringe (a vacuum). It is lower than the atmospheric pressure, causing the gas to expand inside the marshmallow and therefore causing the marshmallow to increase in size. The troposphere contains approximately 78% of nitrogen (N2 ), 21 % of oxygen, 0.9% of argon (Ar), 0.03% of carbon dioxide (CO2) and other gases.


Oxygen (chemical symbol O, Latin name Oxygenium) is a gaseous element which makes the second biggest part of the Earth´s atmosphere. Oxygen is a colourless, odourless and tasteless gas, which is the end product of the photosynthesis. In the air oxygen occurs as a two-atom molecule O2 or as a three-atom molecule O3 which is called ozone. Oxygen is a biogenic element, its existence is essential for the existence of most living organisms on our planet. Oxygen dissolves in water and acts as a strong oxidizing agent.

In the 15th century , Leonardo da Vinci was studying the properties of air and he found out that one of its components supports combustion. Nowadays, we know that it is oxygen. Carl Wilhelm Sheele discovered oxygen in 1772 and named it “fire air”. However, he did not immediately publish his discovery so the credit for it was given to Joseph Priestley, who discovered it two years later – in 1774 and published his work in the same year.

Oxygen and burning (experiment)

In order to prove the importance of oxygen in burning and the proof of oxygen itself, we will carry out a simple experiment. We will need a beaker or a glass, a teaspoon, some matches, a bowl or a plate and tap water.We will fill up the plate with water and place the burning candle into it. Paraffin, that the candle is made from, has a lower density than water therefore when we place the candle on the water surface carefully, it will not sink. After covering the candle by the beaker or the glass, we can see that the level of water inside the beaker is lower than the level of water outside in the plate. The flame of the candle heats up the air inside the beaker and it expands. We can observe air bubbles leaving the beaker. When paraffin is burning, the oxygen, which is in the beaker, is being consumed. When all the oxygen is consumed, the flame goes out. At the same time, the heated air cools down. Cold air has a higher density than warm air, therefore its volume starts getting smaller causing the water to be sucked from the plate into the beaker. At the end of the experiment, the water level in the beaker is above the level of water in the plate. Prevention of oxygen access to burning objects is used when extinguishing fires.

Carbon dioxide

Carbon dioxide (chemical formula CO2) is a chemical compound composed of one carbon and two oxygen atoms. Carbon dioxide is a colourless, odourless and tasteless gas. It is the end product of the respiration processes of living aerobic organisms. It also enters the atmosphere as a gas which derives from burning of coal, wood and other organic substances and fermentation.Carbon dioxide is a non-combustiblegas with a higher density than the air that is why it stays near the ground. Winemakers are well aware of this property of carbon dioxide. In the room where wine ferments, for example when making partiallyfermented grape must, they light a candle and place it to the lower height than they breathe in. If the candle goes out, it warns them that the concentration of carbon dioxide is too high and they must leave the room, otherwise, they would suffocate.

In a lab, carbon dioxide is mostly prepared by reactions of carbonates and hydrochloric acid (Reactive scheme 1) or by reaction of baking soda and vinegar (Reactive scheme 2) in the apparatus for lab preparation of gas (picture 1).

Reactive scheme 1: Reaction of calcium carbonate with hydrochloric acid

CaCO3 + 2 HCl → CaCl 2+ H2O + CO2

Reactive scheme 2: Reaction of baking soda (Sodium bicarbonate) with vinegar(Acetic acid)

NaHCO3 + CH3 COOH → CH3 COONa + H2O + CO2

Fig. 1: Apparatus for a lab preparation of carbon dioxide 
(source: )

We can prove the non-combustibility of carbon dioxide and its bigger density than the one of the air by a simple experiment with 4 burning candles of various heights. We will place the candles into the pneumatic bath and we will start pouring carbon dioxide onto the candles from the apparatus for a lab preparation of carbon dioxide (Fig. 2). The candle which is the shortest, will go out first, the longest candle will burn the longest.The non-combustible properties of carbon dioxide are used when extinguishing fires. Carbon dioxide is the fire-extinguishing agent in snow fire extinguishers. These fire extinguisherscan be used only in well-aired rooms. Limewateris often used to test for carbon dioxide. Limewater is a pure clear suspension of calcium hydroxide diluted in water. Bubbling carbon dioxide through the limewater, a white milky precipitate is caused by the formation of calcium carbonate (Reactive scheme 3).

Reactive scheme 3: The reaction of limewater with carbon dioxide

Ca(OH)2 + CO2 → CaCO3 + H2O

In the exhaled air the content of oxygen is decreasing from 21% down to roughly 17% and the content of carbon dioxide is increasing from 0.03% up to 4%. Further, the concentration of water vapour in the exhaled air is increasing. Water vapour can be proved by exhaling on a cold object, for example a pocket mirror or a glass. The amount of nitrogen stays the same. In order to test for carbon dioxide in the exhaled air, the reaction with limewater is used (Reactive scheme 3).