Matter can be classified in two basic ways: according to its physical state (such as gas, liquid and solid) and according to their composition (as an element, compound or a mixture).

A sample of matter can be a gas, a liquid or a solid. These three forms of matter are called states of matter. The states of matter differ in some of their simple observable properties. A gas has no fixed volume or shape; is adapted to the volume and shape of its container. A gas can be compressed to occupy a smaller volume or can expand to occupy a larger volume. A liquid has a discrete volume independently of its container but has a specific shape. Finally, a solid has a specific shape and therefore a defined volume. Liquids and solids cannot be compressed significantly.

The properties of the states of matter can be understood at the molecular level. In a gas, the molecules are far apart and move at high speed, colliding with each other and with the walls of the container. Compression of a gas decreases the space between molecules, increasing the frequency of collisions between molecules, but does not change the shape or size of them. In a liquid the molecules are “packaged” together but they still move quickly. This move allows some molecules “to slip” on others making the liquid to flow easily. In a solid molecules are strongly bonded to one another, usually with defined places, so molecules can swing little in their positions, otherwise fixed.

Each substance has its unique set of properties that allow us to differentiate between them. The properties of matter can be classified into physical and chemical. The physical properties can be observed without changing the identity and composition of the substance. These properties include the color, odor, density, melting point, boiling point, hardness, etc. The chemical properties describe how a substance can change or react to form other substances. A common chemical property is flammability, that is, the ability of a substance to burn in the presence of oxygen.

Some properties such as temperature, melting point or density are known as intensive properties because they do not depend on the amount of sample tested. They are very useful because many of these properties are used to identify substances. The extensive properties of substances depend on the amount of sample. This class includes, for example, the mass and volume. Extensive properties are related to the amount of substance in the sample.

The changes experienced by substances can be classified, as their properties, into physical and chemical. During a physical change, the substances change their physical appearance but not its composition, ie, it is the same substance before and after the change. The conversion of a piece of ice to liquid water is a physical change. When the ice melts, it changes state from solid to liquid, but is formed by water molecules. All state changes (for example, from solid to liquid or from liquid to gas) are physical changes.

In a chemical change (also called chemical reaction) a substance becomes another chemically different substance. For example, when hydrogen burns in the air is converted into another substance because is combined with oxygen to form water.