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Reading Selection, Lesson 20

Hydrogen and Oxygen

fireman using water to put out a blaze.

Water is a compound formed when inflammable hydrogen reacts with oxygen. Here, it is being used to put out a fire. Like all compounds, the properties of water are very different from those of the elements from which it is composed.

Water is a compound made up of two elements—hydrogen and oxygen. The characteristic properties of these elements are different from those of water. However, hydrogen and oxygen have some common properties. They are both colorless, odorless gases , and they both readily react with other elements—making them "reactive" elements. But in many ways they are very different from each other.

Hydrogen has the lowest density of all the elements. It is very reactive, which is one reason why it is present in only very small quantities in air. It reacts with oxygen. You reacted it with oxygen when it burned with a squeaky pop. What do you think was made in that chemical reaction?

A welder using an acetylene torch.
When flammable gases such as acetylene are burned in pure oxygen, very high temperatures are produced. This oxyacetylene torch burns a mixture of acetylene gas and oxygen, which produces a flame hot enough to cut or weld steel.
It may come as a surprise to you to discover that hydrogen is the most common element in the universe. The sun and other stars are mainly hydrogen gas. Hydrogen is found in many compounds. For example, all acids contain hydrogen.

Oxygen reacts with other substances. Some of the properties of oxygen were discussed in the reader "Air Heads" (in Lesson 4). Oxygen is needed for burning to take place. Things burn well in oxygen, producing hotter flames. For example, what happened to the glowing splint when it was put into a tube of almost pure oxygen?  Some welding and metal-cutting equipment use flammable gases and pure oxygen to produce the high temperatures needed to melt metal.

Oxygen also reacts slowly with many substances. Many compounds containing oxygen are called oxides. You've already come across the two oxides that are gases—carbon dioxide and sulfur dioxide—but most oxides are solids. In fact, oxygen is the most common element in the Earth’s crust, but most of it is combined with other elements to form minerals that make up rocks.

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