The states of matter—solids, liquids, gases, and plasma—are closely related to the concept of thermal energy, which is the energy associated with the motion of particles. The manner in which particles are arranged and how they move in a substance depends on the state of matter, and this, in turn, influences the amount of thermal energy they possess.
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Solids: In solids, particles are closely packed together in a fixed arrangement. They have very little thermal energy, which means the particles vibrate in place but do not move freely. This tight arrangement gives solids a definite shape and volume. As thermal energy increases (for example, when a solid is heated), the particles vibrate more vigorously, and if enough thermal energy is added, the solid can change to a liquid (melting).
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Liquids: In liquids, particles are still close together but are not in fixed positions; they can slide past one another. This increased freedom of movement allows liquids to have a definite volume but no definite shape, as they take the shape of their container. Liquids have more thermal energy than solids, which allows the particles to overcome some of the attractions holding them together. When more thermal energy is added to a liquid, it can change to a gas (evaporation or boiling).
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Gases: In gases, particles are much farther apart and move freely and rapidly. This high level of thermal energy results in weak intermolecular forces, allowing gases to expand and fill the entire volume of their container. The pressure and volume of a gas are also affected by thermal energy—when gases are heated, their particles move faster, increasing pressure if the volume is kept constant. If sufficiently heated, a gas can become plasma.
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Plasma: Plasma is a state of matter where gases are energized to the point that electrons are stripped away from atoms, resulting in charged particles (ions and electrons). This state has a high amount of thermal energy and is typically found in extreme conditions, such as in stars. Plasma can conduct electricity and respond to magnetic fields, differing from other states of matter.
As temperature increases, thermal energy is transferred to matter, causing changes in the state of matter through processes such as melting, boiling, and ionization. Conversely, reducing thermal energy can lead to condensation, freezing, or de-ionization, changing a substance back to a less energetic state.
In summary, the states of matter are characterized by the arrangement and motion of particles, which are influenced by their thermal energy. As thermal energy changes, so too can the state of matter, illustrating the crucial relationship between these concepts.