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Oxygen

by Ron Kurtus (3 December 2007)

Oxygen is a chemical element with the atomic number of 8. It is a colorless gas that is essential for life as we know it. Being slightly soluble in water, it also supports water-borne life. Oxygen combines with many other elements by the process of oxidation. It is the most abundant element found on the Earth and makes up 21% of the atmosphere. As a compound, oxygen is present in water, in plants and animals, and in much of the solid material that makes up the earth. Thousands of compounds contain oxygen, along with carbon, hydrogen, and other elements. Oxygen can be prepared by heating certain oxygen compounds, through electrolysis or by liquefying air.

Questions you may have include:

• What are the properties of oxygen?
• Where is oxygen found in nature?
• How is oxygen prepared or separated?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Properties of oxygen

The atomic number of oxygen is 8, meaning it has 8 protons in its nucleus. Its atomic weight or atomic mass is approximately 16 for the most common isotope of oxygen (¹⁶O) that makes up 99.76% of the oxygen found in nature. Other stable isotopes of oxygen are ¹⁷O and ¹⁸O.

(See Isotopes for more information on that subject.)

Oxygen is a colorless, odorless and tasteless gas that is slightly heavier than air. It is essential for sustaining the lives of all living things.

Oxygen gas is always seen as an O₂ molecule. The oxygen atom is almost never seen by itself. In some cases, three atoms of oxygen will combine form ozone (O₃). Ozone is important in the upper atmosphere to prevent harsh ultra-violet rays from harming living beings. Close to the Earth, ozone is considered an irritant to breathing and part of air pollution.

It is only slightly soluble in water. Although only a small amount of oxygen dissolves in water—about 4 parts of oxygen to 100 parts water. That quantity of oxygen in the water is sufficient to for the vital needs of the vast number of living things that inhabit our oceans, lakes, and rivers.

Although oxygen is a gas at normal temperatures, it may be liquefied by extreme cold. At a temperature of −183° C, oxygen changes into a pale-blue liquid. At −219° C liquid oxygen becomes a bluish-white solid.

When heated sufficiently in air, many materials will combine with the oxygen in the air and burn. If placed in pure oxygen, the burning and oxidation can be quite intense. Examples of common materials combining with oxygen include:

C + O₂ → CO₂ (carbon dioxide)

S + O₂ → SO₂ (sulfur dioxide)

3Fe + 2O₂ → Fe₃O₄ (iron oxide or rust)

(See Oxidation for more information on that subject.)

Occurrence of oxygen in nature

Oxygen is the most abundant of all elements found in nature. It occurs both as a gas and also as part of a large number of compounds.

Oxygen gas is found mainly in the atmosphere, which contains approximately 21% oxygen by volume, along with 78% nitrogen and small amounts of other gases. Some oxygen is also found dissolved in water.

As a compound, oxygen is present in water, in plant and animal substances, and in the solid material that makes up the earth. Thousands of compounds, such as starch, cellulose, sugar, fat, and proteins contain oxygen united with carbon, hydrogen, and other elements.

Enormous quantities of oxygen are combined with elements that make up sand, limestone and other rocks and materials in found in the earth.

Preparation of oxygen

The discovery of oxygen was made in 1774 by Joseph Priestley when he heated the red oxide of mercury (mercuric oxide or HgO) to create a small amount of the gas.

2HgO → 2Hg + O₂↑

Note: O₂↑means it is a gas

Laboratory methods

One common method to create or prepare oxygen in the laboratory involves the decomposition of potassium chlorate (KClO₃). When potassium chlorate is heated, it first melts and then boils, giving up its oxygen. The equation for this reaction is:

2KClO₃ → 2KCL + 3O₂↑

Another laboratory method involves action of water on sodium peroxide (Na₂O₂) yielding oxygen and sodium hydroxide (NaOH). Since this reaction can be violent, the water is allowed to drip slowly from a funnel upon solid sodium peroxide in a flask. The method of collecting oxygen is passing it through a tube and bubbling it up in a collecting bottle filled with water.

2Na₂O₂ + H₂O → 4NaOH + O₂↑

Commercial methods

One commercial method of preparing oxygen involves the electrolysis of water, which consists of breaking up the water compound by the means of an electric current. A dilute acid, such as sulfuric acid, is added to the water to make it a conductor of electricity. Direct current is then passed through the solution, and the water is decomposed into oxygen and hydrogen gases.

The oxygen collects in a water tube containing the positive (+) electrode (anode) at the hydrogen collects in a water tube containing the negative (-) electrode (cathode).

2H₂O → H₂↑ + O₂↑

Another commercial method involves the distillation of liquid air. When air is applied with high pressure and low temperature, it can be liquefied. As it is allowed to warm up, nitrogen—which has a lower boiling point than oxygen—escapes as a gas, leaving nearly pure oxygen as a liquid. It can then be stored either as a liquid or as a compressed gas in metal containers.

Summary

Oxygen is a colorless gas that is essential for life. It is slightly soluble in water and also supports water-borne life. Oxygen combines with many other elements by the process of oxidation. It is the most abundant element found on the Earth and makes up 21% of the atmosphere. As a compound, oxygen is present in water, living things, and much of the solid material that makes up the earth. Oxygen can be prepared by heating certain oxygen compounds, through electrolysis or by liquefying air.

Answers to Readers' Questions

Resources

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Mini-quiz to check your understanding

1. What is the reaction of combining sulfur and oxygen called?

2. What is the chemical formula for common sand?

3. Can electrolysis of water create oxygen in the laboratory?

If you got all three correct, you are on your way to becoming a Champion in Chemistry. If you had problems, you had better look over the material again.

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