Classification of Matter

Classification of Matter

Matter is classified by state (solid, liquid or gas) or by composition (element, compound or mixture).

States of Matter

All substances can exist in one of these forms: solids, liquids, or gases. These three forms of matter make up the states of matter. Their properties are described on the macroscopic level. Gases have no fixed shape or volume, conform to the shape and volume of their container, and are compressible. Liquids conform to the shape of the container, but have a fixed volume and are incompressible. Solids have both shape and volume that are independent of the container, and are also incompressible.

These properties can be understood at the molecular level. In a gas, the molecules are far apart, move at high speeds and collide often. In a liquid, the molecules are closer than in a gas and move rapidly, but can slide over each other. In a solid, the molecules are packed closely in definite arrangements.

Pure substances

Forms of matter with fixed composition that does not vary from sample to sample are pure substances. Substances are either elements or compounds.

Elements

Elements are substances that cannot be decomposed further into simpler substances, i.e. they are composed of only one kind of atom. There are 114 elements presently known. Between 1868 and 1870, Mendeleev created a table or chart that listed the known elements according to increasing order of their atomic weights. We will discuss in greater detail the reasons they are arranged in this manner later in the course (Chapters 6 and 7); we will also see the significance of the numbers above and below the symbol of each element in Chapter 2. In this course, you will find a link on the Course Menu on the left sidebar to a periodic table.

Elements are abbreviated by symbols, consisting of one or two letters. Many of the names are derived from the English language, while others are based on Latin origins. Many of the elements in the lowest part of the periodic table are named for individual or places.

Take a few minutes to test your knowledge of nomenclature for some common elements and their symbols.

Common Elements and Their Symbols

A small number of elements constitute the bulk of the earth's crust as well as the human body, as shown in the figure below.

Compounds

Compounds are made of two or more elements in a definite ratio. For example, a water molecule contains two hydrogen atoms and one oxygen atom. Properties of compounds differ from properties of their component elements (e.g., water is liquid, while hydrogen and oxygen are both gases at the same