Classification of Matter

How are the four states of matter described?

Scientists classify the four states of matter according to their distinct properties:

• Solids contain particles that are closely packed, and vibrate in place. Solids have a definite shape and definite volume. Solids are essentially incompressible.
• Liquids contain particles that are loosely arranged and can move past one another. Liquids take the shape of their container, and have a definite volume. Liquids are essentially incompressible.
• Gases contain particles that are widely separated and are very free to move in all directions. Gases have an indefinite shape and an indefinite volume. Gases are compressible.
• Plasmas, which in nature are found in stars and in space, consist of positively charged ions and free electrons. Plasmas are produced at temperatures greater than 5000 °C.

How are pure substances and mixtures distinguished?

Scientists classify matter as a pure substanceglossary term (opens in a new window) or a mixtureglossary term (opens in a new window):

• A sample of a pure substance consists of the atoms of a single elementglossary term (opens in a new window) or the molecules of a single compoundglossary term (opens in a new window).
• A mixture consists of two or more pure substances that are not chemically combined.

What is a mixture?

Scientists define a mixture as:

• A system made up of two or more pure substances that are combined physically, but not chemically.

How are different types of mixtures distinguished?

Scientists classify mixtures broadly as homogeneousglossary term (opens in a new window) or heterogeneousglossary term (opens in a new window):

• Homogeneous mixtures have a uniform composition throughout. Solutions are examples of homogeneous mixtures.
• In a solutionglossary term (opens in a new window), the soluteglossary term (opens in a new window) is the substance that is dissolved, and the solventglossary term (opens in a new window) is the substance that does the dissolving.
• Heterogeneous mixtures do not have a uniform composition throughout. Suspensions and colloids are examples of heterogeneous mixtures.
• The particles in a suspensionglossary term (opens in a new window) separate over time.
• The particles in a colloidglossary term (opens in a new window) do not separate over time.