States of Matter
Matter can be defined as anything that occupies space. This would include all of the material that makes up our universe. On earth, matter is found naturally in three states, or structures, including solids, liquids and gases. In space, a fourth state of matter known as plasma is more typical.
Since the beginning of history, man has attempted to understand the changes that go on in the environment all the time. To this end, scientists and philosophers developed different theories to explain the nature of matter. The ancient Greeks were particularly prolific in this area. One of the first people to provide a theory of matter was the Greek philosopher Anaxagoras around 500 B.C. He suggested that each substance had its own "seeds" that came together to form substances. Later, Empedocles postulated that there were only four basic seeds or elements including earth, air, fire and water. He believed that these elements were combined in various proportions to make all things. Around 384 B.C. Aristotle added to the basic elements the idea of qualities including heat, cold, dryness, and moisture. When two qualities were combined, they formed the elements. For example, when cold was mixed with wet the result was water. When wet was mixed with heat, the result was air.
Interestingly, Democritus, who pre-dated Aristotle, proposed that all matter was made up of tiny particles. He thought these particles had great voids of space between them, and he called them atoms, meaning indivisible. This theory was very similar to what we believe today about the composition of matter. Unfortunately, Aristotle rejected this theory and it was forgotten about for almost 2,000 years.
Our current understanding of matter was generally developed during the early nineteenth century. Using the work of many scientists before him, John Dalton proposed the atomic theory. This theory postulates that matter is composed of small, discrete particles called atoms. He suggested that these fundamental particles, which made up different substances, had different masses and sizes. This was different than the theory proposed by Democritus who suggested that each fundamental particle was the same. In subsequent years, experiments have supported Dalton's atomic theory. Today, we believe that matter is organized as atoms, ions or molecules.
One of the basic properties of particles of matter is that they can assume different arrangements in space for any particular substance. For example, they can be relatively close to each other or far apart. They can be randomly arranged or neatly ordered. Another property of matter is that it is always in motion. This motion can range from a rigid vibration in a tight position to long range distances in a straight line. Movements can be changed when particles of matter collide. It is the arrangement of particles and their motion that determines whether they will exist as solids, liquids, gasses or plasma. In general, all substances can exist in these states and changes from one state to another represents only a physical change.
Solids are a form of matter which occupy a definite amount of space. They have a rigid shape that resists change. The particles that make up solids are highly attracted to each other and have an ordered structure in space. Their motion is typically limited to vibrations in a limited volume. In general, most substances become solid when they are cooled. This slows the motion of the particles of matter and allows them arrange themselves in a more orderly fashion. Depending on the temperature, solids may be compressed but only slightly. Solids may exist in the form of crystals which have geometric shapes. These would include materials such as salt, sugar, or ice. Less regular solids have amorphous structures. This includes things such as clay, polystyrene, or polyester. If a solid is substantially heated, its particles may vibrate with enough energy to break the rigid structure. When this happens, the substance melts and is converted to a liquid. The temperature at which this occurs is the melting point.
Like solids, liquids occupy a fixed amount of space. Also, they can only be compressed slightly. However, they generally do not have rigid shapes and will acquire the form of the container in which they are held. In liquids, the particles are arranged farther apart than in solids. They can also move about space randomly, although they always maintain contact with some of the other liquid particles. Examples of naturally occurring liquids include water, milk, alcohol, and mercury. If the temperature of a liquid is increased enough, the substance will vaporize and change to a gas.
Gases do not take up a set amount of space and they have no shape. They are made up of particles that are freely moving in space and will uniformly fill any container in which they are held. Unlike solids and liquids, gases can be compressed into very small spaces. Natural examples of gases include air, oxygen, helium, and neon.
Plasma is a fourth state of matter which is closely related to gases. The component particles in a plasma are different from those of conventional states of matter. They are made up of electrons and positive ions. This state of matter is typically found only when the temperature of a substance is extremely high. At these temperatures, the electrons from the atoms that make up the substance are removed, and they move about freely like gas particles. Plasmas are generally only found in space making up things such as stars and comet tails. Much remains unknown about the nature of plasmas and they represent an exciting area of research. Currently, plasmas have been used to make lasers.
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