Water is composed of the elements hydrogen (H) and oxygen (O). When electricity passes through water, bubbles of oxygen and hydrogen gas form and rise to the surface of the water. If the gases are collected in a container and a flame is brought near the mixture, the hydrogen and oxygen react and form water. Water is classified as a compound. A compound is a substance that is made from two or more simpler substances and can be broken down into those simpler substances. The simpler substances are either elements or other compounds.

The properties of a compound differ from those of the substances from which it is made. For example, oxygen and hydrogen are gases at room temperature, but water is a liquid. Hydrogen can fuel a fire, and oxygen can keep a fire burning, but was doesn not burn or help other substances to burn. In fact, water is one of the substances commonly used to put out fires.

A compound always contains two or more elements joined in a fixed proportion. For example, in silicon dioxide (SiO2), there are always two oxygen atoms for each silicon (Si) atom. (Di- means "two.") In water, there are always two hydrogen atoms for each oxygen atom.


  • Elements have different properties than their compounds. Silicon is a gray solid and oxygen is a colorless gas, which can be stored in a metal tank. Silicon and oxygen combine to form silicon dioxide - a colorless, transparent solid found in most grains of sand.

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