Physics
States of Matter
Intermolecular Forces and States of Matter
11
⚡ Quick Summary
The state of matter (solid, liquid, gas) depends on the separation between molecules and the strength of intermolecular forces. In solids, molecules are close with strong forces, in liquids they are farther apart with weaker forces, and in gases they are far apart with very weak forces.
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If two molecules are kept at a separation r = r₀, they will stay in equilibrium. If they are slightly pulled apart so that r > r₀, an attractive force will operate between them. If they are slightly pushed so that r < r₀, a repulsive force will operate. Thus, if a molecule is slightly displaced from its equilibrium position, it will oscillate about its mean position. This is the situation in a solid. The molecules are close to each other, very nearly at the equilibrium separations. The amplitude of vibrations is very small and the molecules remain almost fixed at their positions. This explains why a solid has a fixed shape if no external forces act to deform it. In liquids, the average separation between the molecules is somewhat larger. The attractive force is weak, and the molecules are more free to move inside the whole mass of the liquid. In gases, the separation is much larger and the molecular force is very weak.