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Properties of states of matter:
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Solids have a fixed volume and shape.
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Liquids have a fixed volume but no fixed shape.
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Gases have no fixed volume. They spread out to fill all the space available to them. Gases have least density. Unlike solids and liquids, gases can be compressed.
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Kinetic theory of matter: According to this theory, molecules of solids, liquids and gases are in continuous random motion.
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In solids, molecules vibrate about fixed positions and they have minimum space between them. There are strong intermolecular forces between molecules.
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In liquids, molecules are not fixed in their positions. Molecules have slightly more space between them compared to solids. Intremolecular forces are weaker than in solids but still fairly strong.
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In gases, molecules have high kinetic energy and the intermolecular forces are negligible. Average distance between molecules is greatest.
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Evidence of molecular motion
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Diffusion
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Reflections from smoke particles in a glass cell are observed using a microscope. The smoke particles show random motion because of the air molecules bombarding them.
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Observing pollen on the surface of water
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Kinetic theory of matter can be used to explain a range of phenomena including:
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diffusion
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evaporation
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pressure of a gas
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fixed shape of a solid
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expansion
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