All matter in the known universe is composed of atoms. Each substance has a distinctive atomic structure, made up of a particular combination of protons, electrons and neutrons. The state in which this matter exists is called the phase of the matter. There are four phases of matter in which everything on Earth can be explained. Matter can be either a solid, a liquid, a gas or a plasma.
Matter in a solid phase is made up of atoms and molecules that are closely held together by molecular forces. Solid matter is rigid in shape, and that shape determines its volume. The atoms within a solid vibrate, but that small and rapid movement is often difficult to detect. Solid matter can be either crystalline or amorphous. The atoms in crystalline solids tend to be packed tightly and held in place by atomic bonds. Examples of crystalline solids include quartz and salt. Amorphous solids can exist in either a "rubbery" or "glassy" state. Window glass is an example of an amorphous solid.
The molecular forces in liquid matter are not as strong as those in solids, so a liquid can flow, despite having a definite volume. Due to this flowing, a liquid typically conforms to the shape of the container in which it is held. The only exception to this is microgravity: In this state, a liquid inside a free surface forms into a ball rather than taking the shape of the surface. Liquids are similar to solids in that their particles touch and their densities are close to those of solids.
Matter in a gaseous phase has no definite shape or volume. If confined within a container, that is the shape it will take. Otherwise, a gas will spread out indefinitely until meeting some sort of resistance. Unlike a solid or a liquid, the particles in a gas are separated enough to result in a low density. Liquid water that is heated above 100 degrees Celsius, or 212 degrees Fahrenheit, will turn to vapor, making it a gas. Other gases include nitrogen, oxygen and the helium used in balloons.
Only recently did scientists begin to study the fourth state of matter, plasma, an ionized gas. Plasma is matter that can be found in extremely high temperatures, such as those found on the sun or created during a spacecraft's re-entry from beyond Earth's atmosphere. Outside of Earth, up to 99 percent of our visible universe, as well as much of that which is not visible, is made up of plasma. In plasma, the protons, neutrons and electrons all float freely, in stark contrast to solids and liquids. Because of their low density, interparticle collisions in plasma are unlikely to occur. Plasmas can also be found on Earth in flames, lightning and auroras.
The "Fifth State"
In the 1920s, Satyendra Nath Bose and Albert Einstein predicted a fifth state of matter. This fifth state is now referred to as Bose-Einstein condensation and was only recently discovered. This condensation refers to the collapse of atoms into a single state, which is essentially the opposite of how they behave in a plasma. Bose-Einstein condensation will only occur in extremely low temperatures, such as near absolute zero. These temperatures are too low to be produced anywhere on Earth outside of a laboratory.
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