The molecules are closely arranged in solids, loosely arranged in liquids and are free to move in gases... But, Why are liquids (especially water) exhibiting these distinctive properties such as incompressibility, surface tension and wetting phenomena?
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Liquids are a state of matter in which the atoms or molecules are held together by chemical bonds (a difference from gases) but the bonds are weak enough for the shape to be variable (a difference from solids). That's why it is not possible to increase or decrease their volume much; the amount of energy from these chemical bonds would rapidly increase because the "optimum" volume is what minimizes the chemical energy. That's why the liquids are approximately incompressible. On the other hand, they may change the shape, by definition, because the chemical energy between pairs of atoms or molecules may be replaced the chemical energy of another pair as long as the volume is the same. The role of the chemical energy in the overall energy is much larger, relatively speaking, than it is for gases. However, there's really no universal reliable procedure to distinguish gases and liquids. There is typically a phase transition between them which allows one to distinguish the phases at a given temperature. But if one goes e.g. above the critical point of water (very warm; high pressure), the phase transition becomes smooth i.e. it disappears. The wetting and surface tension comes from the contribution to the energy from the chemical bonds on the surface that would be inside the liquid but they're not because it's the surface. Such corrections inevitably exist for all fluids, including gases. It's just harder to notice it for gases because gases penetrate everywhere, anyway, and do everything they can to increase their volume. For liquids, the total energy contains volume-extensive terms and the surface terms, $$ E = \alpha V + \beta A $$ where $A$ is the area. One may only accurately determine the coefficient(s) $\beta$ because the volume-extensive term is fixed; liquids are mostly incompressible. However, for gases, the first term becomes variable because $V$ does and the second term therefore plays a smaller role in the qualitative behavior of the substance. |
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