Description

Matter is a substance that has inertia and occupies physical space. According to modern physics, matter consists of various types of particles, each with mass and size. The most familiar examples of material particles are the electron, the proton and the neutron. Combinations of these particles form atoms. There are more than 100 different kinds of atoms, each kind constituting a unique chemical element. A combination of atoms forms a molecule. Atoms and/or molecules can join together to form a compound. Matter can exist in several states, also called phases. The three most common states are known as solid, liquid and gas. A single element or compound of matter might exist in more than one of the three states, depending on the temperature and pressure. Less familiar states of matter include plasma, foam and Bose-Einstein condensate. These states occur under special conditions.

Different kinds of matter can combine to form substances that may not resemble any of the original ingredients. For example, hydrogen (a gaseous element) and oxygen (another gaseous element) combine to form water (a liquid compound at room temperature). The process of such combination is called a chemical reaction. A chemical reaction involves interactions between the electrons of the atoms, but does not affect the nuclei of the atoms. In some situations, matter is converted into energy by atomic reactions, also known as nuclear reactions. This type of reaction is fundamentally different from the chemical reaction because it involves changes in the nuclei of atoms. The most common example of an atomic reaction is the hydrogen fusion that occurs inside the sun. The immense pressure inside the sun, and inside other stars, forces atoms of hydrogen together to form atoms of helium. In this process, some of the mass is converted to energy according to the formula

E = mc ²

where E is the energy in joules, m is the mass in kilograms, and c is the speed of light, which is approximately 2.99792 x 10 ⁸ meters per second in a vacuum. In recent years, scientists have confirmed the existence of a substance called antimatter. The electron has an antiparticle twin called a positron, with equal mass but opposite electric charge. Similarly, the proton has an antimatter twin called an antiproton, and the neutron has an antimatter twin called an antineutron. If a particle of matter encounters its antiparticle, both are converted entirely to energy according to the above formula, where m is the combined mass of the particle and the antiparticle. Small amounts of antimatter have been isolated in laboratory conditions, but no one has yet succeeded in creating a controlled a matter/antimatter reaction, or even an uncontrolled reaction of significant size.

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.

• 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. Relationships between states of matter This diagram shows how solids, liquids and gases – the three classical states of matter – can interconvert.

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.