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Solids: Introduction

Introduction: States of Aggregation

To understand the significance of the crystalline state, it is advisable to recall the specific characteristics of the three states of aggregation: solid, liquid, and gaseous.

Fig.1
Gas (Ep<<Ek)
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Fig.2
Liquid (EpEk)
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Fig.3
Solid (Ep>>Ek)
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Ep = potential energy, Ek = kinetic energy

The gaseous state

Fig.4
Steam
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In the gaseous state the atoms or molecules are completely mobile. The sum of the interaction energy between the particles (Ep) is considerably less than the kinetic energies (Ek) of the molecules or atoms. Consequently, the gas particles are largely free to move around. They occupy the entire available volume enclosed by the walls of their container.

The solid state

Fig.5
Ice
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This contrasts with the solid state. The solid state of aggregation is characterized by the fact that the particles are packed relatively densely and - except for vibrations - are locked in place. There are considerable attractive forces between the particles (Ep>>Ek). Despite their superficial appearance, most solids are crystalline, meaning that the building blocks within the crystal are in a regular arrangement. This arrangement is referred to as a crystal lattice. Macroscopically, crystals have smooth boundary surfaces. Solids that do not have a regular structure are amorphous. Examples of amorphous solids include glasses and minerals like flint, opal, and obisidian. The fracture planes of these substances are bumpy. In their Debye-Scherrer x-ray diffraction pattern, crystalline materials give sharp lines whereas amorphous solids give diffuse shadows.

The liquid state

Fig.6
Water
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The liquid state lies between the solid and gaseous states. The sum of the attractive interactions between the particles (Ep) is only slightly greater than or equal to the total kinetic energy (Ek). The system adopts the shape of its container walls (flow). Depending on the conditions, a greater or lesser number of molecules are constantly moving from the liquid state to the gaseous state (evaporation).

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