Matter is anything that occupies space and has mass. All physical objects are composed of matter, and an easily observed property of matter is its state or phase. The classical states of matter are solid, liquid and gas. Several other states, including plasma and Bose-Einstein condensate, do exist, but it is the classical states that can transition directly into any of the other classical states.

For example, an ice cube (solid water) left on a bench at room temperature quickly changes to liquid water, whereas a jet of steam (gaseous water) from the spout of a boiling kettle changes to liquid water when directed onto a cold surface.

Classifying matter

Another way of thinking about matter is from the chemist’s viewpoint. The following diagram shows a way of classifying matter with elements and compounds very much in mind.

To see how this classification system works, let us take two examples:

  • How does milk fit into this system? Milk has a uniform composition, and it is a solution (homogeneous). Filtering the milk will separate out suspended solids (proteins and lipids), leaving behind a clear liquid. The application of various separation techniques to this liquid yields numerous categories of chemical compound. Milk is a complex mixture of substances.
  • How does golden beach sand fit into this system? Most of the grains of golden beach sand are either light-coloured feldspar or clear quartz. Beach sand has a non-uniform composition, but the grains can be physically separated into mineral groupings. Sand minerals are chemical compounds.

Kinetic-molecular theory of matter

Solids, liquids and gases each have their own characteristic properties. In order to explain how these properties come about, the kinetic-molecular theory has been developed.

According to this theory, all matter is made up of extremely small particles (atoms, molecules or ions), which are in constant motion:

  • In solids, these particles are tightly packed together, usually in a regular array, and vibrate back and forth.
  • In liquids, the particles are still tightly packed, but as well as vibrating, they can move over and in between one another.
  • In the gaseous state, the particles are spaced out relative to one another and are moving around with rapid, random motion.

For each of these states, the higher the temperature, the faster the particles move.

Relationships between states of matter

The classical states of matter (solid, liquid and gas) can transition directly into any of the other classical states. Either adding heat to the system or removing heat from the system can achieve this.

Two other states of matter

When a gas is heated to very high temperatures, the gaseous atoms are stripped of their outer electrons. This creates ‘ionised’ gas that consists of a highly energetic mix of positive ions, electrons and atoms. The gas has been converted into plasma, often referred to as the ‘fourth state of matter’.

In the 1920s, two scientists, Satyendra Bose and Albert Einstein, predicted the existence of a state of matter at the extreme low-energy end of the temperature scale. They called this state ‘Bose-Einstein condensate’. There are only a few selected elements and subatomic particles that can reach this state. It was not until 1995 that two scientists, Cornell and Weiman, were able to make this condensate from atoms of rubidium.

Nature of science

In scientific terms, a theory implies that something has been proven and is generally accepted as being true. Scientific theories are well established, highly substantiated, internally consistent systems of explanation. The kinetic-molecular theory of matter provides such a system of explanation for the observed properties of matter.

    Published 29 April 2014