Chapter 10 - Solids Liquids and Phase Transitions The differences between solids liquids and gases is significant but for many(but not all

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Solids, Liquids and Phase Transitions The differences between solids, liquids and gases is significant, but for many (but not all) properties: Solids ≈ Liquids, but very different from gases The differences in bulk properties for different states of matter is fundamentally a reflection of how far apart the molecules are which is a consequence of the nature and magnitude of the intermolecular forces. The qualitative nature of these forces is always the same, however, irrespective of the phase: long range attractive
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Molar Volume Molar volume of a gas is always ≈ 24,000cm 3 mol -1 (Avogadro’s hypothesis) For liquids and solids the molar volume is typically 10-100cm 3 . When a solid melts it typically changes volume by only 2-10%, which is very small compared to the change on going from liquid or solid to gas. The structure of liquids, solids and gases and the average distance between molecules reflects these differences: Solid (3-5 x 10 -10 m) Liquid (3-5 x 10 -10 m) Gas (30 x 10 -10 m)
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Compressibility Not surprisingly gases are far easier to compress than either solids or liquids. The compressibility of a gas is 2 atm -1 while for solids and liquids it is 10 -5 -10 -6 atm -1 . This is a reflection of the much greater distance between molecules in the gas phase, which influences the nature of the forces between molecules. Gases Solids-Liquids
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Thermal Expansion If we heat a gas at constant pressure its volume increases rapidly, but the changes in solids and liquids are almost imperceptible. (e.g. when a gas has its temperature increased by 1 ° C the volume increases by 0.4%, while for solids and liquids it is on the order of 0.02%) As we increase the temperature we give the molecules more energy so they tend to be further apart from one another. The difference between liquids, solids and gases is that in liquids and solids strong attractive forces must be overcome for the atoms to move further apart, while for gases the fact that the atoms are far apart from one another means that the attractive forces are much weaker and therefore increases in temperature produce comparatively large changes in volume. Gases Solids-Liquids
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Diffusion The diffusion constant measures the rate of diffusive mixing (D). For this property: D(gas) > D(liquid) > D(solid) and liquids can be thought of as having properties that are truly intermediate between solids and gases
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Diffusion of Solids vs. Liquids Atoms in a solid vibrate about fixed positions due to strong intermolecular forces Atoms in liquids are in well-defined local structures which persist for only short periods Solid Liquid A melting crystal
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Surface Tension Surface tension arises because the forces on atoms/molecules at the surface are not isotropic. Consequently, atoms at the surface exhibit a higher “free
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This note was uploaded on 01/20/2012 for the course CHEM 030.101 taught by Professor Draper during the Fall '08 term at Johns Hopkins.

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Chapter 10 - Solids Liquids and Phase Transitions The differences between solids liquids and gases is significant but for many(but not all

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