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States of Matter
Unless the temperature of a substance is absolute-zero (0 K = -273 °C), its particles will have some amount of kinetic energy. This means that the molecules that make up a substance are in constant motion. The physical state of the substance, or its phase of matter, depends on the motion of its particles. Changing the temperature of the substance will change the motion of the particles, which will affect its physical state. Matter is usually classified as one of three main states: solid, liquid, or gas; though a fourth state, plasma, is sometimes identified.
The particles of a solid are usually packed closely together and vibrate in place. They are often arranged in a fixed pattern, meaning that while they do move as they vibrate, they do not generally move out of their positions relative to one another. This means that solids usually keep their shape (fixed shape) and have a consistent volume (fixed volume). Substances tend to form solids toward the lower end of the temperature scales. As the temperature increases, however, the particles will gain more kinetic energy. Once the temperature passes the substance’s melting point (equal to the freezing point), its phase will change from solid to liquid.
Particles in a liquid tend to be less tightly packed than in their solid form (though there are exceptions such as ice and water). They are able to flow past one another without any regular arrangement, meaning that liquids usually have a fixed volume but an indefinite shape. As a result, liquids will take the shape of the part of the container they are held in. Some liquids, such as water, will flow more freely than others, such as oil. A liquid’s resistance to flow is called its viscosity. Liquids with high viscosity like honey flow slowly; liquids with a low viscosity like milk flow easily. If the temperature of a liquid is raised, its particles will continue to move faster and faster until the substance reaches its boiling point (equal to its condensation point). At this temperature, it will transition from a solid state to a gaseous state.
The particles of a gas are very energetic, moving freely and spreading far apart to take up all of the available space. As a result, gasses tend to assume both the shape and the volume of their container (indefinite shape and volume). Gasses tend to occupy the higher end of the temperature scales. However, if enough energy is added to a gas (super-heated) it can become ionized, meaning that some electrons are freed from the gas molecules creating ions. Ionized gas is called plasma and can be observed in fire, lightning, and stars. Interestingly, because stars are considered plasma, this means that the overwhelming majority of the matter in the universe exists in a plasma state.
