Solids | Research & Encyclopedia Articles

Solids

Solids represent one of the three states of matter, liquids and gases representing, respectively, the other two. A solid has a fixed volume, high molecular density, a definite shape, and it foes not flow. These characteristics are exactly opposite to those of a gas. A liquid could be described as an intermediate form. A solid can change into a liquid by melting, a liquid changes into a solid by freezing, and some solids can change directly into a gas by sublimation. The temperature at which each of these events occurs is a fixed property of the solid. Chemists know these temperatures for pure substances; however, if the solid is not a pure substance, these temperature are harder to determine. In fact, an unexpected melting point is an indication that a substance is impure. The melting point and the freezing point are identical. A substance with a high melting point has strong intermolecular forces.

In a solid, the molecules are in close contact, and there is little intermolecular movement. In addition, a solid has a high degree of regularity in molecular arrangement.

When a solid is heated there is very little expansion due to the effects of heat. Similarly, when a solid is placed under pressure there is very little change to the solid. The particles constituting a solid are held rigidly in place and can only vibrate. As the temperature increases, they vibrate more vigorously, remaining rigidly in place until the liquid phase is reached.

While a solid usually has a much higher density than a liquid, this is not true for water. Solid water, or ice, is less dense than water. This is is a crucial feature, because it enables ice to float on water. This, in combination with other features of water, has been instrumental in the development of life on this planet.

The inner structure of a solid depends on the type of bonding that exists between the solid's constitutive molecules. As a general rule, regardless of the bonding, the particles tend to form the closest packed arrangement of the largest particles present. Solids with a highly ordered structure are said to be crystalline in nature. For example, particles of metal will form a lattice in regular arrangement.

A solid can be dissolved in a liquid. When this occurs, the solid is called called a solute, the liquid is a solvent, and the result of the process is a solution. Any solid will not dissolve in any liquid. If a solid is an organic compound, then it will generally only dissolve in an organic solvent. Similarly, if the compound is inorganic, an organic solvent, such as water, is needed. Sometimes a solid will not dissolve even if the appropriate solvent is used; in that case, the solid is said to be insoluble.

Kinetic theory explains the distribution of molecules and molecular energy within a solid. The intermolecular forces holding the particles together in a solid are strong enough not just to hold them in together but to virtually hold them in place. The intermolecular forces present within a solid may be powerful enough to fold the molecules in position but they are much weaker than the ionic bonds that are present. Less energy is required to melt a solid than is needed to break the bonds in the molecules. Because of the closeness of the particles in a solid, a solid (and a liquid) is sometimes referred as the condensed phase.

Solids can occur in a crystalline or non-crystalline (amorphous) form. Crystalline solids have their particles in well-defined and ordered arrangements. Such solids usually have flat surfaces or faces, adjoining surfaces making a definite angle. A diamond is a perfect example of a crystalline solid.

Amorphous solids are fundamentally different: their particles are not structured in an orderly manner. These solids do not have well-defined shapes or angles. Most amorphous solids are mixtures of particles that do not stack together well, and this may be because they are composed of large, complex molecules, or due to a considerable inbalance between the sizes of the molecules. Rubber is an example of an amorphous solid. If a crystalline solid with a regular arrangement is heated to a sufficiently high temperature, some of the molecular bonds will break. This temperature is usually higher than the melting point. If the liquid is then cooled rapidly, there is no opportunity for these bonds to re-form. When the solid is re- formed, the molecules are no longer able to align in an ordered manner. Consequently, the formerly crystalline solid becomes amorphous. Due to its lack of an orderly structure, an amorphous solid does not have a definite melting point.

Because a crystalline solid is regular, we can see the inner form of the entire solid by looking at a fragment. The smallest repeating unit of the solid is the unit cell. A larger unit is a crystal lattice. The crystal lattice is a three-dimensional array of atoms. The principal lattice structures are simple cubic, body-centered, and face-centered. The simple cubic consists of eight atoms forming a cube. The body-centerd form is a cube with a ninth atom in the center. In the face-centered cubic, there are six atoms added to the eight of the basic cube, one for each face. Regardless of the lattice composing a solid, the particles are always placed in the closest possibly proximity, maximizing the attractive force between them, so they hold the solid's shape as rigidly as possible. This is known as close packing.

The arrangement of particles in a crystal can be seen by using a technique called x-ray diffraction. This technique projects x rays through a crystal and records the beam on photographic film. If the crystal is sufficiently thin, the researcher can infer the particle arrangement from the picture produced.

It is the bonding in a solid that is responsible for the solid's specific characteristics. For example, molecular crystals are held together by a variety of weaker forces; as aresult, these solids are soft and have a lower melting point. Strong solids are formed with a structure called a covalent network. Diamond, graphite, and sand (silicone dioxide) represent this class. Their molecules are bonded to each other in layers. Another type of bonding is found in ionic solids, exemplified by sodium chloride (NaCl), or table salt, crystals. Here, the particles are held together by ionic bonds, each ion being surrounded by several oppositely charged ions within the inner structure of the solid.