Pure Substances
Matter that always has exactly the same composition is classified as a pure substance, or simply a substance. Table salt and table sugar are two examples of pure substances. Every pinch of salt tastes equally salty. Every spoonful of sugar tastes equally sweet. 
Every sample of a given substance has the same properties because a substance has a fixed, uniform composition. Substances can be classified into two categories—elements and compounds.
Elements
Although there are millions of known substances, there are only about 100 elements. An element is a substance that cannot be broken down into simpler substances. Imagine cutting a copper wire into smaller and smaller pieces. Eventually you would end up with extremely tiny particles called copper atoms. An atom is the smallest particle of an element. An element has a fixed composition because it contains only one type of atom.
No two elements contain the same type of atom. In Chapter 4, you will find out more about atoms, including how the atoms of one element differ from the atoms of every other element.
Examples of Elements
At room temperature (20°C, or 68°F), most elements are solids, including the elements aluminum and carbon. You have seen aluminum foil used to wrap food. Most soft drink cans are made from aluminum. Carbon is the main element in the marks you make with a pencil on a piece of paper. Some elements are gases at room temperature. The elements oxygen and nitrogen are the main gases in the air you breathe. Only two elements are liquids at room temperature, bromine and mercury, both of which are extremely poisonous. Figure 2 shows four elements and their symbols.
Figure 2 Aluminum, carbon, and gold are elements that you can see in common objects, such as cans, pencils, and rings. Mixtures containing iodine are used to prevent and treat infections. Analyzing Data Which of these elements has a symbol that is not related to its name in English?
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